Titanium is much picker than steel about being welded. Most steel welds are done with CO2 shielding gas or a CO2/argon mix; argon is less commonly used because of the cost, although it's sometimes used to help control overpenetration in thin sections. Titanium, as you point out, requires argon or helium, and also requires better joint preparation and a very clean work environment. Also, since titanium requires shielding down to around 800 degrees F, the power source must be equipped with a secondary contactor so that the shielding gas can be maintained around the weld zone as the weld cools - you can't simply pull the TIG torch away, as the weld will oxidize.

Steel frames are typically TIG welded because of the precise heat control that's possible with TIG, allowing thin sections to be welded well with a minimum of filler metal. MIG welding tends to be more economical for larger weld sections due to its much higher metal deposition rate. With care, SMAW (stick) and FCAW (flux-core wirefeed) processes can be used on frames, although these aren't used by any commercial framebuilders that I'm aware of.

FWIW, one of the principal advantages that carbon has over other materials is the ability to adjust the stiffness in different directions separately. With a good layup schedule, you can build a frame that's laterally stiff but vertically compliant by varying the directions of the fibers in various areas of the layup. Steel and titanium have the same stiffness in every direction, so the framebuilder is kind of stuck with trying to make the frame laterally rigid without giving up all the vertical compliance that everyone wants.