Is it reasonable to write the major part of my statement of purpose on machine learning for masters in data science?
Actually, the SOP should look to your future, not the past. Your CV tells what you have already done, so you don't need to repeat that. But if you want to talk about machine learning, talk about how that background applies to your future in data science. If you intend, for example to work at the intersection of the two fields, talk about that. 

But in general, you want to convince the admissions people that you have realistic goals and are well suited to achieve them. The main focus should be forward looking, not backward.