The main reason it's not enabled is as a spam preventative measure. It's far from uncommon for spammers to post a series of seemingly legit threads with no links, and then edit the posts to include links a few days later. Whilst we can monitor new posts easily, we can't monitor edit changes for every post made by new members to make sure a spam link hasn't been included.
The other concern is, as Digerati has stated, that new members will inadvertently misuse it. For a lot of people, posting here will be the first time they've ever used forum software. It's easy to get confused about what is/isn't relevant and should be edited out, likely leading to confusion and hard to follow threads. This isn't an issue for most forums, but as we're a support forum we need threads to have a clear structure and not be missing information.
A large number of users won't even notice the edit feature, let alone misuse it, but there are more drawbacks to enabling it for everyone than there are benefits. As such, there are no plans to enable this feature to all members, but we're considering other alternatives.