Whilst I think many visitors would be pleased to see Spotted Hyenas, they are deemed to be of Least Concern by the IUCN (a huge tribute to the species' versatility and capacity to cope with change), and thus they don't appeal to zoos seeking to "do their bit". And they can be very destructive: a carnivore with such powerful jaws, that can dig, and is very inquisitive, is a greater challenge to keep than many other similar sized carnivores.
Personally I think it's a huge shame: I'd far sooner see a clan of Spotted Hyenas at Whipsnade than yet another pride of generic zoo lions.