This may enlighten as to why there's such a difference (timestamp provided in the link).

https://www.youtube.com/watch?v=YRZ_4H0OK60&t=529

The TL;DR of it is that 1.0 had a huge focus on graphics fidelity at a high cost in production and performance, limiting both the the ammount of playable content offered (while the market, mainly WoW, had changed in pro of offering more playable content) and the max ammount of models that could be displayed on screen at once (which isn't ideal for an MMO).