To avoid regulation in an era of moral panics.
The 1950s was a pretty socially conservative time. Showing some ankle was scandalous. Broadcasters refused to film Elvis below the waist because his dancing was too lurid. They were pretty uptight. There were also some pretty gory, violent comics for adults that would even challenge our modern, relatively libertine morality. Lots of people were freaking out about the perversions that their children might be exposed to. Comics burnings were organized and Congressional hearings were held. It was looking like Congress might regulate or even ban comics as some localities already had done.
So publishers decided to be proactive and self-censor. They created the Comics Code Authority and appointed some of the moralizers to it to give the moralizing public confidence that their comics were "safe". Comics that passed CCA review were given a stamp, kind of like the MPAA rating system. Distributors refused to carry comics that weren't approved. At first the restrictions were pretty strict, (police must always be respected, criminals must always be caught, no Hollywood monsters like vampires, werewolves, or zombies. You couldn't even have a black lead character!) Even the comic that birthed The Mighty Thor, Hela, and the rest of their comics pantheon, Journey into Mystery, was originally a horror comic until the CCA forced its format to change to fantasy. Without the CCA, there might never have been a Thor or Hela!
Though the code was eventually relaxed and finally abolished, originally, writers and publishers were even stricter than the code required. They wanted to show that they were adhering to it in good faith. In particular, invoking the devil was not allowed. They named a satanic character Mephisto to get around this, for example. So there was no way they were going to feature a character named Hel, goddess of the underworld. That is just way too Satanic for the 1960s. So they renamed her Hela.