Well, the Banshee honestly probably had a very very short lifespan. It came into limited production towards the end of the great war, and we're talking the very tail end. And it was essentially an anti-shield weapon with a bit of hull damage, horribly energy inefficient and generally specialized entirely for taking on ships with powerful shields and weak hulls, ie, the shivans. After the lucifer was destroyed, and the shivans disappeared, there was no reason to expand the banshee manufacturing capabilities - the only reason it exist had vanished. Against shivans it was a useful weapon. Againts terran and vasudan space pirates....well, it's not cost or energy efficient.
The Prom-R is harder for me to explain, primairly cause of order of events. If they were developed post great war and then replaced by the subach, I could understand it. However, that's just not the way it's shown in FS2. Prom-R comes after the subach, and it seems quite unlikely to me they'd give subaches to new recruits if they were only in limited production - and they sure wouldn't make you wait until you're flying a heavy assault fighter to get the PoS-R. And the tech database claims the subach sidearm is the standard weapon from the beginning of FS2. There are certainly no shortage of them laying around.
*shrug* the only real possible explanations I can come up with for the Prom-R are:
A:) Someone said to command "This civil war has been going on for quite some time, could drag on longer, or worse, could degenerate into a second round of the Terran-Vasudan war. We've got all this old Prometheus manufacturing equipment around, and we've figured out how to make a working, if not wonderful, weapon with it. Better to be prepared just in case, neh?"
B:) Someone said to command "Hey, I want to sell you a bunch of lasers. Come visit me on my space yacht, prostitutes and liquor on me, including some ultra-rare real earth vintages."