two main reasons
1.because fewer people play open queue. (top 500 in OQ starts at like 3000 sr last i saw)
2. people dont really care about there rating in open queue, largely its used by DPS mains too practice heroes with faster queue times.
as a result of these two things; theres no benefit to play open queue if your aiming for diamond+ because almost none of us play it, it wouldnt make sense for instance for a top 500 tank main too play open when they can do role queue.
if you play in a given rank in both role queue and in open queue, you’ll notice that the open queue players are alot worse. aka easier to kill and beat which leads too the SRs not matching there role queue mirror (like plat to plat or gold to gold).
like a match at 2500 in open queue geninuely felt like playing with people at 2000 in role queue. due to the lack of competition, the open queue SR’s are inflated greatly (i believe this is why some people ONLY play open queue, as its there highest SR)
theres also alot more smurfs in open queue, because its a throw away rank. the only place thats going to be populated is the low elo, bronze-gold maybe plat. so people can throw matches and smurf freely without the 10+ minute queue times. AND its “free” on an already existing account, being 1500 on open doesnt effect your main role sr.