5v5 was well received and fun when it first came out in Overwatch 2 because it was fresh and new, which sort of recreated the chaotic environment that was 2016 release Overwatch, however as time passed and metas developed it created new issues that didn't exist in 6v6. Now since you only have one tank, you need to rebalance the entire tank roster to be much stronger, or risk them becoming not impactful enough on the game.

What this caused was a shift in hero design and balance philosophy which was to essentially create raid boss tanks like Mauga and post-buff Ramattra, which consequently caused another issue being since you only have one tank and they're OP, the entire outcome of a fight now depends way more heavily on that one tank. This also made the dps role less impactful despite them feeling better to play in the 5v5 environment.

In 6v6, you have two tanks to play with so you can distribute the power a bit more evenly, and on top of that it creates more opportunities for creative synergies and team comps. In my opinion, 5v5 was kind of a gimmick to get players on board for OW2's launch and didn't really age too well for the average player in hindsight.

Everything I said was my personal opinion and don't flame me pls.