They conquered larger, more powerful, more unified and more distant lands, but not India - why?
I hate to be this guy, but I feel like I should point out that this question has been asked before! Le me direct you to these threads. Someone else might say some more in this thread, but those would probably be good places to start.
Since we have a rule against simply posting links, I'll say that it seems the consensus is that the Mongols were very reliant on cavalry, and the mountainous terrain of India made their tactics pretty nonviable. There were also problems with supplying food for their horses. Finally, India was in fact conquered by the Mughals, which are related to the Mongols.
hi! in addition to the thread linked by /u/LuckyRevenant, there's a section on this topic in the FAQ (see link on the sidebar)