Poker: why is the WSOP main event held sometimes in July and sometimes in November?

It is not as random as the question makes it sound. The Main Event is a series of games that culminates in a final table of nine players which determines the tournament winner. In 2008 that final table was moved from June/July (when the preliminary contests were still held) to November, when it was held for several years before the managers decided to revert to July. The reason for both changes was television.

Prior to 2008, the entire Main Event was played without interruption. Starting in 2008, in an effort to build excitement in the WSOP and to increase ratings for the tape-delayed televised shows, Harrah's Entertainment and ESPN decided to delay the final table until shortly before its scheduled broadcast. The delay would allow ESPN to cover the rest of the tournament leading up to the final table without viewers knowing the winner in advance. Due to the timing of U.S. presidential elections, the final tables for the 2012 and 2016 Main Events were held in October.

In 2017, Poker Central announced a television and digital media rights agreement with the WSOP and ESPN which returned the final table to the tail end of the rest of the tournament.

https://en.wikipedia.org/wiki/November_Nine