The bike in my avatar, a Bike Friday Air Glide, uses a titanium beam instead of the traditional seatpost/seat tube arrangement. The rest of the frame is a much more conventional cromoly alloy.
My understanding is that all the beam bikes they produced, including the Air Friday, were discontinued when Green Gear could no longer source those titanium beams for whatever reason: cost, availability, quality, etc. And since the titanium beam is critical to the bike's design and function, those models are gone, perhaps never to return. Glad I got one.
Now extrapolate that to an entire bike frame. As in jur's post above, titanium requires careful handling to prevent weld contamination, which could eventually lead to weld failure, and we don't want that happening.
Long story short, I don't think it's as easy to produce a titanium frame as some folks think.