Basically they wanted lots of money/ advertising rights for various companies to have press conferences at E3 proper so all three big publishers said fuck that and held their press conferences around the same time and still in LA but not actually part of E3.

Iirc from 2019, xbox, play station, Nintendo, Bethesda, square enix, and the PC games press conference were all held the week before. Those shows and the associated panels are the biggest draw of E3 and almost none of them showed up.