Mio said that FubuMio's been a thing ever since she joined hololive, but they would typically do more or less FubuMio collabs depending on how they felt the popularity of their pairing was doing. Like, it'd come in waves and if it seemed like people were excited about them they'd do more and if not they'd just hang out with others more.
And that was kind of the tempo they had up until Mio's hospitalization. At first, Mio saw herself as someone who lagged behind everyone else in Gamers, and saw everyone else as members who could carry the event without her. She initially figured that the event would be fine even without her, and she kinda rationalized things that way to herself for a bit. That is, until she talked more with Okayu after the hospitalization and learned from Okayu that Fubuki was actually devastated by Mio not being able to participate. She was so depressed about Mio not being able to be there she initially felt something akin to "There's no point in having Gamers Fes without Mio, why even bother with it?" It took some talking to her to convince her that if they ever wanted to give Mio a chance to actually appear in a Gamers Fes, they'd have to work to make that one a success.
Mio then said after being so down about her not being around, Fubuki was extremely delighted to see her back and mentioned again how she ran figure eights around her in excitement after knowing she was free from the hospital. Since then, Fubuki has been a lot more proactive in inviting Mio to do things and contacting her, which led to Mio really understanding how much she meant to Fubuki, and thus leading Mio to feel like she really doesn't want to make Fubuki sad and wants to treasure her as well.
Mio noted it's like they both finally have their "arrows pointed at each other," and their feelings more mutual. So that's why FubuMio seems to be undergoing a boom nowadays.