Smaller in scope thus simpler. Unmaintained. Far easier to borrow pieces from as needed.

For example I implemented networking on top of my Impact game with relative ease (networked games are never easy) while layering it on top of my Phaser gamer would've taken longer and with more uncertainty because Phaser is far more opinionated and larger in scope.

IMHO - Phaser3 is promising to be the winner html5 framework last few years. But I didn't check if anything new came up in past 1 year.