To root a phone, it has to be booted into at least temporary rooted mode, because the file that has to be copied to it has to be copied to a system folder, and you can't do that without being rooted. There's usually some exploit that can be used (which gets patched in the next update), but someone has to have the phone and try hundreds of different things before finding something that works.
And that's not likely to happen unless the phone is popular enough that finding how to root it will benefit a lot of people. So if you have a pohone not owned by many people, there's very little chance of anyone buying one just to find a way of rooting it.
Don't be fooled by these apps/programs that claim to root many different phones. They do - but not all in one program. The program searches the phone to find out what it is - and that can be down to the country and carrier - then contacts the developer's server to download the software needed to root that particular phone on that particular version of Android. And they're using the same rooting methods that you'll find on
XDA Forums. That's where most of the developers who work on rooting phones hang out. So if your phone isn't even listed there, there's just about no chance of finding any way of rooting it.
Simple enough?