Two reasons: TIG is slower, and TIG needs more skill. The welder feeds filler rod with one hand and runs the torch with the other, watching the puddle the whole time. A weld that takes ten minutes in MIG might take thirty in TIG.
The result is worth the cost when the job needs a clean weld, a food-safe finish, or a thin-material weld that won’t burn through. For structural work behind a panel, MIG is usually the better economic call.