What is the most important factor in making a language hot?

Many programmers use and want to use that language.

What makes a language easy?

First of all, easy is a comparative term.  When we say language A is easy, we are actually saying language A is easier than tool B.  So an easy language lets a programmer or developer complete a project faster or with less headaches then any other tool currently available.  Let us examine the problem of evaluating an algebraic function for one value.  I could write a program in C, or use a calculator.  I think most of use would use the calculator.  The calculator is better for the task, and for this project a calculator is easier then C.

If experienced programmers keep languages hot, then why is COBOL not a hot a language?

Experienced Programmers are only one of the components.  I do think that COBOL remained a hot language long after it should have stopped being used.  Oddly we don't have a language that replaced COBOL.  The gap that COBOL has left is being replaced by tools, not another language.  These tools are composed of high end database programs.  Ask yourself a question.  If you were going to set up a database for a small to medium size company, what would you use, Access or COBOL?

Can an easy to learn language be substituted for an already known language?

I think this depends greatly on the scope of the project.  Even easy to learn languages take some time to learn.  If the project is long learning a new language may be in order, but if the project is short, the already known language may be the best choice.