Maybe because they ran out of space? Nathan said they had to cut out a lot of the equipment chapter to stay within their limits, so the same probably happened to mutations.
Besides the "Heretics & Mutants" book will go into depth with mutations, so I wouldn't worry about it. Most likely half of the book will be all about mutations.
Personally to me mutations are a small and unimportant element of the setting.
MC3 homebrewer. Check the thread!
CA-138 Deathlockdrum - when you want to put a grenade launcher on a grenade launcher, so you can launch grenades when launching grenades.