My best guess at the reason for this is that there are many more DPS mains than support or tank players.
The skill rating are basically always going to show as an odd bell curve because any Elo system is actually a skill comparison system - not a skill rating system. Not many people understand how to play tank and support well, so the average skill is much lower in those roles whereas DPS has a ton of players, so the average DPS player is better at their role than the average tank or support player.
I believe that’s only for new accounts. I’ve never seen this on my main account but have seen it on alts.
Sidenote: My fresh MMR account got placed 3 ranks too low, so I don’t exactly trust this system all too much.