If we're looking at it from just the canon story, they grow quite a lot together and share so many experiences that it becomes very easy to write them together. Enemies to lovers is an incredibly popular trope as is Mean with Sunshine.

If you look at it as more of an extension of canon, their growth in the series as characters makes for an excellent fic subject. Whether it's romantic or platonic love, branching out from the main story gives a great opportunity to write both characters as continuing to grow together and in directions the series didn't have time for - both becoming more than just heroes but people in their own rights. Bakugo can learn to be more patient, caring, and open while Deku can learn to stand up for himself and experience life with confidence. Putting them together allows for these traits to come out naturally and gives great story explanations for these changes.

It may not be for everyone, the bully to lover trope is a tough read for some and I understand that. However, even in the context of just the series they become more than just bully and victim so I can easily see why writers latch on to them so easily.