Because on the rear, HS33 needs more maintenance, setting, grinding imo. I grind my rear wheel more often than my front wheel on my dual HS33 stock. Disc on the other hand holds way better and is more sought.
Also, I don't know why, but with a front disc, on a mod, I can't do any plants on the front. But I do them a lot with front HS33 though. That's just a confidence problem but I hate front disc.