I've addressed this several times already... But I'll try for a little bit different perspective here:
Keeping it simple
There are two common models for editing in most widely-used software packages. One doesn't track changes at all (at least not publicly), while the other tracks every change. Let's call them the Forum model and the Wiki model:
Forums and message boards grew out of email and USENET, and as such many didn't (and still don't) offer any sort of editing support: you post your message, it enters the timeline and stays there, static, forever. The big downside here is that most of us make lots of mistakes, and seeing those ensconced on the 'Net FOREVER kinda sucks.
So some forums let authors (and maybe moderators) make edits, at least for a short while after the message has been posted. Generally though, there's no revision history to speak of, beyond perhaps a simple "edited at [time] because [reason]" footer. If I post something like,
Obama has forever ruined Life in America as we know it.
...then wait for you to respond and change it to,
I like waffles!
...there's not much you can do to prove I wrote something else. If a moderator, drunk on power as is their nature, goes in and changes it to,
I like waffles, but don't have any. THANKS, OBAMA!
...then there's not much I can do about that either.
Generally speaking, forums have a pretty crappy (or non-existent) editing culture.
The wiki model
This is the other extreme: every revision is kept for an indeterminate amount of time; if I want to see what you changed when you revised your post at UTC 7:31, I can look that up. If I want to see what you changed at UTC 7:32:19, I can look that up too. If I want to see everything that changed on a post between UTC 5:00 and UTC 12:59, there are tools that'll let me select a range of revisions to compare as well.
The advantage here is that nothing is hidden - if someone decides to play silly games, their shenanigans will quickly be noticed by one and all. As such, it's easy for a robust editing culture to develop: no one needs to be afraid of Being Bold with their edits, since there's no chance of inadvertently attributing their work to someone else.
The disadvantage is that it quickly becomes very noisy. If I want to see what's changed on a given page since the last time I looked at it, I might have to walk through multiple revisions - perhaps dozens or more - before I'm able to get a clear picture of what happened. The tools get more complex to combat this; pretty soon, someone implements a "minor edit" feature to allow hiding revisions that aren't really that interesting... Which then starts to erode the advantage outlined above.
Stack Exchange edits
The Stack Exchange model for editing is a hybrid of these two approaches. While it falls somewhat closer to the wiki model than the forum model, it deviates from the latter in a few key areas:
Edits made by the same editor in a short period of time are collapsed: no matter how many times a post is edited within a 5-minute window, only one revision is stored (as long as only one editor is involved). This largely eliminates the need for a "minor edit" feature.
Tools for comparing revisions are much more limited: only revisions directly adjacent in the chronology can be compared.
Outside of Community Wiki posts, an original author is always maintained and clearly identified even if substantial changes have been made by other editors since the post was created.
Together, this hybrid system allows for a much simpler, much easier-to-read, easier-to-navigate user interface. However, this comes at a cost: trivial one-off edits must be discouraged in favor of more comprehensive edits.
If merely maintaining a simple UI were the only concern, it might be worthwhile to consider an optional "expert" mode that allowed minor edits at the cost of a more involved UI. However, there's a much bigger concern...
The cost of ownership
As much as it can act like a wiki at times, Stack Exchange posts are not wikis. The most important distinction is the one I noted above: nearly all posts have a clearly-identified author, whose name and image always appear prominently near it, whose account gets full credit for the post and whose reputation score increases or decreases according to how the post is received by the community.
The advantage of this was perhaps debatable in the early days, but has since become clear: folks will put an awful lot of effort into building their personal portfolio. Selfish? Sure - but we all benefit from the results.
The downside of this is... Well, folks can get a bit tetchy when you mess with something they consider a reflection of their expertise or personality. Early on, 3rd-party edits were a very controversial part of the system; even today, the notion that someone - anyone - can come in and mess with your words can be very disconcerting to new users.
There were wars... Fought over very minor edits.
And it quickly became clear that it is hard to sell someone on the value of 3rd-party edits when all they see is pedantry. If someone takes your barely-legible, heavily-downvoted question and turns it into an easy-to-read, easy-to-answer showpiece, well... The proof is in the pudding! But when someone else's name suddenly appears on Your Masterpiece and you look only to find that they've been obsessively adding Oxford commas and nothing else... Well, shucks - that can be a bit hard to swallow.
Conclusion: the system gently encourages behaviors that make life easier for all concerned
No, that wasn't a title; that's the actual conclusion here. There are other factors involved that I haven't touched on, but you can find them linked to in the first paragraph.