Severity of concussions plays into it.
I don't know how bad Randy's was in 2011 but one of the ones that Ziggler had he admitted not remembering the rest of the match and didn't fully come to until the next day. He also stated that he had severe headaches that he thought were never going to go away.
Given that Ziggler has had more than one he himself stated that for his one in 2014 he stayed out longer even after he was cleared because he wanted to give it a little more time.
He had one in 2013.
A couple years ago NHL player Sidney Crosby had a really bad concussion and suffered from post concussion syndrome so he stayed out for a long time. Quite a bit longer than most NHL players do.
Randy may have had a concussion but it could have been a minor one, which is why they have testing in place. Same as the NHL and same as the NFL.
Again Corey Graves had a couple and that's why he's a commentator and pre-show host now.
Ex NFL players didn't sue the league over MCLs, ACLs and neck injuries now did they? They sued over post concussion syndrome related incidents.
Orton is quite a bit more popular than Ziggler. So while he definitely is a liability, the WWE would be more willing to take a chance on him.