Lara + substandard gameplay = franchise, like the blood and guts from Mortal Kombat + substandard gameplay = franchise. There's no way that Mortal Kombat would be a franchise if it had the same MK gameplay, but the same look as Street Fighter 2 rather than being a gorefest... I mean just look at all the 16-bit fighters that didn't have a gimmick like that but did have substandard gameplay. How many of them turned out to be franchises?
Uncharted has the gameplay and the storytelling. If it had little to no story whatsoever, mediocre gameplay, and the same male lead, it probably wouldn't be a franchise. In that case, if you took out the male lead and put some boobalicious chick in skimpy/tight clothing, it would have a better shot at being a franchise.
A lot of it has to do with timing too. Mortal Kombat was released at a time where ultraviolence wasn't common in games. The original Mortal Kombat predated Doom by over a year, and came out just a few months after Wolfenstein 3D did... that's how far back we're talking about. Violence is certainly more common in games today. Lara is a bit of a mixed bag. I don't think female leads were ultra common back then. Further divide that by skimpy clothing and massive tits, and stick it in a style of game that wasn't common at the time(or maybe perhaps the expansiveness of the game, plus gameplay perspective), and it becomes somewhat unique. To me it was the gimmick more than anything that made those games franchises that still live on today.