Titanium is currently more expensive than aluminum, due mostly to the refining process. (USGS Metal prices shows Aluminum at a little over $0.60 a pound vs. Titanium at around $4 a pound.)

Titanium is more difficult to machine than Aluminum.

Obviously, flashlights and other items could be made from titanium, but the added cost wouldn't be worth it in most cases. I'd love to have a Ti HDS EDC, but it'd cost $$$ for something that would only be a little lighter.

Now, the titanium scales on my Sebenza are worth it, since they're large hunks of metal that'd make the knife much heavier in anything else.