Ti is an expensive raw material. More importantly it’s a bitch to weld compared with steel or ali. This drives up the skill requirement (and therefore price) and also takes longer (drives up the price) and this gets it above a threshold which limits the numbers made, making it more expensive (development costs get spread over less bikes). Also, it can’t easily (read cheaply) be made into complex (ie aero) shapes.
As for longevity, I’m not so sure. Cameron Brown used to ride LIghtspeed Ti frames and cracked them relatively regularly according to scuttle butt around these parts.
I used to Covet a 650C lightspeed Tachyon back in the day…but that was the same time a a “Slingshot” was on the list as well. Oh yeah, and a Softride… Now I just want a P2 (Carbon, not the original P2) to replace my now aging ali P2K