How to tell professor the reason for dropping his/her subject without it sounding rude? In our university, we are allowed to drop or cancel our classes before a certain deadline within the semester. The reason for dropping is, officially, not required but most professors I know usually asks the student why before agreeing since the professor's consent is required. 

I'm planning to drop out since I'm not finding enough time to research and read outside of class hours. Don't get me wrong, it is interesting but I'm currently struggling in my thesis. I want to tell the professor that I'll drop because I don't think I'll be able to focus on his/her subject, especially since it also has a lot of required work outside of class, but how do I do this without insulting the professor or sounding rude? I'm worried he/she might not take it too kindly when the reason given is someone 'don't have enough time for their class.'
Tell them that you need to focus your efforts on fewer things especially your thesis as you find it challenging.

Be polite and don't forget professors have heard it all before, they will understand or should do.