Why is it so hard to convince professors to write recommendation letters for me?
In general, when you ask someone to write a letter of recommendation for you and they are unwilling to do so then it is likely that the reason they are unwilling to write a letter of recommendation is because they don't feel that they can write a positive and helpful recommendation letter for you and they don't want to write a negative recommendation letter.  

There are many reasons why a letter writer might not be able to write a positive recommendation letter for you, including that they don't think you're well prepared for the program, they don't feel that they're qualified to recommend you for a program outside of their area, they don't feel that your record is sufficient to gain admission to the program in question, or perhaps that they have doubts about your intellectual ability or character.  

In any case, it is almost certainly not in your interest to continue to press for a letter of recommendation from them, since you can be pretty sure that the resulting letter of recommendation will not be positive and won't help you in gaining admission to the program that you're applying to.  

In particular, you should be aware that your professor could be much more candid (and negative) in a telephone conversation than they might be in a written letter of recommendation.