Computers are fast these days. Something that it seems is all too easy to forget if you grew up in the 1980's, when home computers were slow, you spent most of the decade programming in BASIC, and weren't even particularly good at it.
In the best Unix terminal emulator, I made the regular expression searching automatically highlight new output. In Edit, I didn't. Even though it's obviously the right thing to do.
In my defense, it was a couple of years ago now, and Java wasn't as fast as it is now, but I'd guess it was fast enough back then. Here's how I know: the matches are updated in real time as you type your search expression. Have been for years. So what did I think would be so expensive about updating the matches as you edit the document?
Dunno. Search me.
[Update: weak puns aside, while actually making the change I came across commented-out code that suggests my original intention was that further editing should cancel the find. Having since used applications that behave in both ways, I prefer the ones that keep find results up to date unless I explicitly tell them to stop.]