Titanium bikes are very rare because for the price you can get very high quality carbon composites, which most people prefer
As for the topic, it’s apropos to our ti tangent. Scandium made the aluminum a bit easier to work and a bit more supple (al frames can be brutally stiff, like really racy carbon frames) I think they triple butted the ends of the tubes to provide enough material to bond which added stiffness. With scandium you can weld it without as much metal, it’s stronger and since you use less metal it has a bit more flex. Al construction got better and carbon came along which is even cheaper... I keep a bianchi veloce cross scandium bike, still quite delightful to ride, it’s my main winter bike and it doesn’t seem to beat me up like some of the really stiff al racing bikes of the era.
Steel is much more forgiving (Ti is the same, iirc), and carbon fiber in a suitable matrix can handle dynamic loads above 50% of it's ultimate tensile strength.
Note: I'm personally excluding modern carbon bars here, because I find the lovely sculpted wings far more comfortable when compared to old-style bent tubes :)
I have 2 carbon mountain bikes that I've put thousands of miles on. Both are about 7 years old at this point. Early carbon as a bit fragile, but newer carbon seems pretty damned solid.
Even if you do break them, they are repairable. My wife broke the frame on her bike not long ago and she wound up getting it repaired. It was surprisingly affordable and the fix is likely stronger than the original.
There is also the old saying "If you don't tell the rest of the group ride that your bike is titanium, it will transform in to aluminum."
Another argument against titanium is that mining and purifying of titanium is not so easy on the environment. If that is important to someone, it might get counted as a factor.
My understanding is that with advances across the industry, it's pretty easy to build a bike under the UCI minimum weight, which suggests that you could make the frame out of whatever is best for the application.
You can get stupid light in Steel, Ti, Carbon, Aluminum, but once you're in the sub 16ish lb range, you're going to be going with superlight parts all around, as there's just not that much frame left to make light.
You can make a fully equipped (steel) randonneur at 20lbs, with fenders, rack, pump, generator, and lights. It's not cheap (~10k+ is my guess), but man is it a well crafted bike. With some care, I think that style bike can be done with common components and off-the-shelf frames in the 25lb range.
Rodriguez is advertising production steel bikes at the 14lb level: https://www.rodbikes.com/catalog/outlaw/outlaw-main.html Yeah, they're pricey. (10k) (That's production vs custom frame, meaning that they might have one off the rack in the right measurements, if not it's a couple of hundred to do a custom geo)
But superlight Carbon is going to be up there too, and it's going to use basically the same parts, +- bottom brackets and such. And frankly, I would trust superlight steel before I'd trust superlight carbon.
There are some pretty awesome Ti bikes as well, including custom 3d printed lugs, cranks, forks, stem, etc. They are in the same weight range, and IIRC 8kUKP.
If you're looking at commercial production, probably fair that carbon is going to be lighter. But the the people pushing the edges aren't limited to that.
But for mountain bikes, the high-end seems to be titanium.
Every major brand uses carbon fiber in their high end bikes.
Last time I bought one ~2011 I swear it was a little steel, mostly aluminum and a tiny fraction of titanium.
It's a custom frame which costs much less than a commercial carbon fibre frame. Titanium is only expensive because there aren't that many vendors that make them, and they all want to position themselves as high-end.
For both road bikes and mountain bikes, Ti is a vanity product. Carbon dominates mountain bike industry at every price-point above about $3,500 right now.
There are certainly a few high end ti bikes, but carbon is king and very few people still argue ti is superior in terms of performance.