Why increment isn't popular online blitz? I find it strange that the dominant time controls for online blitz is 3 min and 5 min instead of something with delay like 3 increment 2. Fide blitz tournaments have increment too. Any insight on this trend?
The main reason, I think, for online blitz sessions not including increment/delay is the possibility of premoving. You can't premove in OTB chess, so you will use the increment for the 'motion' time.

Personally, I feel 3+0, where you have to keep good track of your clock, is more challenging than 3+2, where you just have to liquidate to a winning endgame using your time more freely and then play on the increment until checkmate. In sudden death games, you can flag your opponent if you are losing, so once they get a winning position it is not game over :)