What is more important developing a new business customer or increase the revenue in existing business which is more profitable for an organization?

Both are important, however what matters most for an organization is to be able to adapt to ever changing business needs of the market which makes it competitive and stay relevant.

Customers are the major reason for the existence of a business and customer relationship are the most important aspects in business today. It is better to develope, maintain & enhance lasting relationships with customers. Without customers business ceases to exist, therefore customers need to be handled with extra care. This will result into the "increased revenues" that every company wishes to have

Increasing revenue from an existing customer is more important for any organization.

At the end of the day, business has to generate revenue and thereby consistent profits.

From a new customer, we cant expect immediate revenue due to its project completion time.