Github was always the go-to option and there was no reason to really even consider alternatives at first. People said "Bitbucket has free private repos!" for years and that was interesting but not compelling. Few users left Github because their competitors had free private repos.

Github got acquired, and that acquisition basically erased any and all user trust in Github as a company because they no longer control their own destiny. Instead we have to trust Microsoft. And that was a big enough shock for people to actually consider all of the competitors instead of just defaulting to Github like they did before.

To some extent, I suspect people paid for private repos from Github because they believed in supporting Github as a company and $7/mo isn't that much. Nobody believes in supporting Microsoft as a company. Microsoft charging for private repos is like Disney having a Patreon account.