Because the installer depends on a lot of upstream utilities (python version, kernel version, dracut interfaces, glibc changes, etc) it can never just be moved from version to version without a lot of work. Just keeping track of all the special cases of hardware X, bios Y needs this can take up a lot of time also. So the team can either work on old anaconda or a new one.. and the release must have a working installer so it is either going to be the team doing old like usual or new.
This has been a problem from RHL 6 timeframe.. (and before with the older installer). And like most coding problems you can't throw more developers at it. The people who know how to write a new code base are the ones who would be the ones who would have to maintain the old. [And while everyone is welcome to help out.. it would seem that only a very very very small number have volunteered to do so.]