Why? Well, the answer is simple: better balance.
Overwatch realized this as well as other games. Now when the que time is way better it really should be time to remove stacking. For ppl who want to participate to tournies it’s stupid to have a game that doesn’t reflect the real tournie games at all. Also it’s frustrating when playing ranked.
It’s fine to have class stacking in unranked because there is no rank but stacking classes really affect the end result of a ranked games if you happen to get a bad or a good stack. Putting every team as close to the opposing team team comp wise would also make the ranking more accurate for everyone.