Why with business analyst vacancies posts, technicall skills are required? Isn't it true that those technicalities should be processed by system analysts instead?

Business analysis as a discipline, has as one of its primary origins, stemmed from the technical world and has throughout the years evolved into the different emanations we currently have. As a discipline though it is the umbrella definition that encompasses all areas of a business/systems continuum viz. Architecture (business, systems etc.), Process (business and systems), Data, Analytics etc.

A business analyst should be able to be as flexible as possible in methodology, tools, design as well as understanding an environment and its complexities deeply. This makes it necessary for the BA to be relatively comfortable in both the business and technical spheres.

First , lets have a fast definition of business analysis

It is the capability of the analyst to understand how the organization functions, and how the processes are interrelated and how one department function affects another. It is helping the organization to cast light on best practices, determine capabilities to improve, or solve a problem. Hence the BA has to know how the stakeholders(internal and external) interact, and doing the right thing for sustainability of to satisfy the business needs. 

Technical needs are required because the organization needs the right thing to be done in the most efficient way with the least cost of change and maximum benefit in short time.  

A technical expert is needed to identify what can be improved, how, who are the people to be engaged, how to determine their needs, how to get consensus for new way of performance,  how to convince people who are accustomed to one way of doing their work to change to another, how to find alternatives, control risk, asses gaps in the process etc... 

Hence when the business analyst, determines the solution ( to a problem ) or to improve a process, sets the plan, the roles and the responsibilities, the relations between departments.... he can sit with the system analyst to determine how this technicalities be implemented in the system. Both complement each others work. 

The Concept of Analyzing the Business itself requires some technical skills to aware the challenges of the technology before introducing to any Business. Example: If your company or client is eligible for implementing the ERP platform then being a Business Analyst you have to suggest the suitable ERP system for the Business Domain. To suggest any ERP product you must aware the Technology challenges over the product which you trying to purchase. System Analyst is something related to System Design aspects but not Business related aspects.