The nice thing about spending money in support of a candidate is that if that candidate withdraws and chooses to back another candidate then most of that campaign money spent is still a net positive.

Whereas, the GOP's entire attack strategy almost certainly focused on just Biden and not a whole lot of policy attacks. With Biden out, that money may as well be a pile of cash they lit on fire. They've also spent quite a bit on content they haven't released yet that is no longer relevant to this election.