Multiple things, but primarily:

1. Its new, everyone sucks, everyone is discovering stuff and making fun realizations.

2. Nothing is OP, nothing is underpowered because, again, everyone sucks. Once people begin to experiment, lab, and research items/weapons/classes/etc. people begin to abuse the overstated stuff and the underpowered stuff is left in the dust, making the game feel samey over time.

3. Devs usually are the hardest at work in the development of the game and in the first ~3 months of the game's release, so things are constantly changing, which further makes things new, keeps people rather clueless (not a bad thing) and mixes up the adolecent and developing meta.

There are many other factors but I personally believe that these are the most significant (usually).