It goes way back to when some 700 MHz LTE spectrum was auctioned off in 2007. Before the auction and before the FCC finalized the rules that would apply to the spectrum the FCC commissioners were circulating drafts of rules they were considering. One of the drafts was proposing open access rules.

Google wrote to the FCC and made a binding commitment to bid at least $4.3 billion if the final rules included certain open access rules. The FCC then included those rules, thus guaranteeing they would get at least $4.3 billion.

Verizon outbid Google and so got that spectrum along with those open access requirements, which included no SIM locking.

In 2018 Verizon, citing fraud concerns, asked the FCC to relax the no locking rule. The FCC agreed, allowing a 60 day lock.