Why do some features go away?

I’m a new user. I notice, while browsing the forums, that there seem to be a number of comments about “it worked in 3, but not in 4.” And the response is “this feature wasn’t ported to the new version.”

My question is, why not? It seems to me that updates should add features and/or fix bugs. Why would other features be deprecated?

I can’t give you specifics offhand; it’s just an impression I get.