There is a very simple and easy answer to your question. People upgraded them. Most of those droids are 4 thousand years old. HK series was created 4000 years before BBY so of course people would have created a new kind of assasin droid that makes the HK series obsolete. (Maybe one that doesn't have to say Statement: at the beginning of every sentence.) The T3 series was eventually upgraded to the R2 series. The C3P0 protocol droids eventually was declared obsolete to another form of protocol droid. Things become obsolete all the time in real life and we as a society just create something new and better to it
Here is my question
Whatever happened to Jaden Korr?
For example after Jedi Academy he was only barely mentioned in a few novels but it was like Kyle never had an apprentice.
"There's gotta be a better way to make a living"