More than likely it is probably because it's an older design. I want to say it came out as competition for the Federal Hydrashock back in the 90s. If I'm not mistaken, I believe it was designed by the same guy who designed the Hydrashock.
Plus, as far as I know, it's never been adopted by a LE agency and that's typically how self defense rounds get their reputations for effectiveness, good or bad. I'm not aware of any actual shootings involving Starfire, though there may be some out there.
It's been a long time since I've seen gel tests with PMC Starfire, but I recall them doing reasonably well. I'm tempted to say they had good expansion but maybe not quite as deeply penetrating as modern rounds. I could be wrong, though.
If it's what you have on hand, they function reliably, and you can shoot it well, it'll probably work fine if needed. Personally, if I were going to buy more self defense rounds for my .38 Special, I'd stick with either Speer 135gr SB-GDHP +P or some version of the FBI load (Buffalo Bore standard pressure being my favorite). I'm not saying those are the only options, but that's what I would choose. I'm not even sure if PMC still makes Starfire ammo. I only use their 132gr FMJ for range practice.