I'm not sure I agree with the comparison here. I used MooTools back in the day, and it was just as easy to use as jQuery to my mind. The syntax was slightly different but not hugely - when I gave into the inevitable and switched to jQuery it didn't take long for me to understand it.

jQuery succeeded because it captured the largest amount of mindshare, particularly important for a library that relies on numerous plugins. Right now Angular is looking pretty similar - it's not pretty, but it is widely adopted.