Reasons were given in
the April 2017 Banned List Announcement in which Protean Hulk was unbanned:
Quote:
On the other hand, we feel as though it’s time to let Protean Hulk off the leash. A number of factors led to this decision. Support within the community has been tilting toward Protean Hulk for quite some time. Inside the Rules Committee, we have been leaning in that direction for a while as well, but didn’t have enough of a consensus. Now we do. We acknowledge that the card will be strong, but are of the opinion that it won’t be the centralizing factor it once would have been. Back when Protean Hulk was banned, both creatures and graveyard control were nowhere near as strong as they are today. We know combo possibilities exist with the card, but they need to be specifically built around, so to us it becomes a great value card instead of a dangerous combo piece in casual environments. We suspect that Protean Hulk will be much like Kokusho, the Evening Star when it was unbanned: powerful but not broken in the current Commander landscape.
In general if you're wondering why a card was banned or unbanned, a good first port of call is to run a search in the
Rules Announcements forum for the card's name.