Why HTML5 technology is not yet so popular and adopted in building websites ?

The problem is that although parts of the language are very stable, the language itself is considered a work in progress, so technically, any of the elements could change at any time.
The language is not expected to be completed for several years, which complicates things further.
Thankfully, a lot of the language is considered stable and ready to use.
I think it's such a great move forward, that you should develop a Graceful Degradation approach to writing your HTML.
That simply means writing HTML that will work with older browsers, but will offer users with more modern browsers an enhanced experience.

Myth #1: HTML5 was invented because Apple stopped supporting Flash .
Apple seems to get blamed for everything, but while HTML5 has some amazing solutions for mobile, it was not created to meet the demands of the iPhone.
In fact, Opera and Mozilla got together back in 2004 to try to solve the problem of a messy web, defining seven principles for good design.
Their goal was simple – propose these principles to W3C as a roadmap for a sleeker, faster, better HTML standard.
The principles covered aspects such as backwards compatibility, error handling, practicality, open development and avoiding device-specific profiling.
It wasn’t until 2006 that the proposal was accepted, and in three short years a new spec was drafted, closed and made ready for use.
Myth #2: HTML5 won’t be ready till 2022.
I’m not even sure how this myth began circulating, but the working draft was closed – meaning nothing more can be added or significantly changed – in 2009.
Because it has been driven by Opera and Mozilla from the start, it may even reach W3C’s Candidate Recognition this year, as almost the entire spec is supported by both browsers.
Even still, you can use HTML5 right now and rest assured it will degrade gracefully and work in older browsers with the inclusion of some lightweight JavaScript, all the way down to complex Canvas implementations.
All current browsers support HTML5.
What you need to be careful of isn’t the markup, it is the CSS.
Even with pre-made CSS resets intended to help out older browsers, design built around CSS3 can fall apart in older browsers unless you take care of the details.
Myth #3: HTML5 requires CSS3.
It makes the most sense to use semantic class names and CSS3 declarations when building something with HTML5, but it isn’t a requirement.
HTML5 is just markup, so it gets along just fine with CSS2 styling, but what it does require is complete styling for all presentational elements of your design.
In other words, the HTML5 spec removes attributes and tags that control the look or style of any element, and is no longer tolerant of inline styling.
The browser is now the supreme governor of how your site is rendered and what rules you need to abide by when creating layout with your markup.
For example, in HTML5 you need a “display:block” declaration for each HTML5 element for them to be understood properly in IE8 and earlier.
In short, HTML5 needs styling to look good, but it doesn’t necessarily have to be CSS3.
Myth #4: HTML5 ruins accessibility.
On the contrary, HTML5 is built around accessibility.
While drafting the spec, utmost care was taken to ensure each element works with WIA ARIA landmark roles.
These roles are specialized attributes added to your tags that allow accessibility devices such as screen readers to better interpret the site’s flow and content better.
Converting a site to HTML5 may ruin the accessibility if these roles aren’t understood and implemented properly, but that won’t be the fault of the markup! See font accessibility.
Myth #5: HTML5 will kill Flash.
The fact is that Flash is still used by several million websites and developers world-wide.
The misconception was born, once again, from the high profile fallout between Adobe and Apple and the rapid adoption of HTML5 audio and video for application and mobile development as a result.
However, though HTML5 is great for low volume video playback, full HTML5 support requires two or three times the encoding chores of Flash support and still lacks many critical features currently available in plug-in based technologies.
Currently, sites like Vimeo and YouTube use HTML5 technology largely for technology’s sake, and will not be moving away from Flash too rapidly.
Developer Viki Hoo points out several other reasons why Flash will be around awhile in her compelling argument here.
But let’s get back to Flash with regards to web design.
In that context, we are likely to see a rapid switch from Flash to HTML5 in the coming years.
HTML5 is easier to learn and use, even for those of use without a high level of JavaScript mastery.
It doesn’t require a 3rd Party plugin to work, loads faster and is extremely mobile friendly.
So it isn’t that HTML5 will kill Flash, it’s that it will dominate the mobile design and development market.

Cross-Browsing compatibility issues

Any programming language that starts off new is difficult to deal with or might have errors.
I believe HTML5 DID solve some issues (For example, displaying flash and interactive modules in mobile devices) but still requires alot of enhancements.
Give it some time and it should definitely do a good job in helping alot of developers out there!

The HTML5 specification is not completed yet.
and unfortunately older versions of IE still have a significant market share.

because it a new technology and some programer they should to training and define this technology

The question itself is not valid for now as HTML5 is actually getting very popular these days.
Initially it was not very popular and the biggest reason as i see was less browser support.
But now things situation has changed a lot as many of the popular browsers have added their support for HTML5.

I think the html5 is being used the last year and this year in huge templates, there are no problem to use it, because many simple tags (like article, section, header, footer) are support by all major browsers, but the problem is that some advanced tags(like canvas) aren't support by old browsers(IE6, IE7 etc) .
but i think if you are following the advanced web, html5 is totally in use because these problems have alternative solutions.
I'm front end developer and i use html5, i have some templates in themeforest and i have many templates online and the html5 and css3 are the future of front developer

IE is the problem :)

HTML5 is still under enhancements and not all browsers supports it yet! you will have to add many special cases for IE's and other unsupported browsers!

Because Adobe flash take a large market.