Even if this is an old thread, I'd add an important reason why Monero is so popular which hasn't been mentioned.

It's a decentralized cryptocurrency with no central "managers". The launch was relatively fair, although there seems to have been some instamine by a selected group. But most other "privacy coins" were much worse. Zcash for example had a premine (which benefitted the founders enormously), Dash had a much larger instamine than XMR, and let's not talk about Bytecoin (Monero's "sister currency").

Until now, the Monero community keeps its decentralized flavour. The only other privacy coin I know with similar "ethics" like Monero is Grin.