Inconel is used in chemical plants for its extreme resistance to harsh chemicals which quickly corrode even the highest-quality stainless steels. Inconel (and its cousin, Monel) is primarily made from nickel (>70% for Inconel-X, >63% for monel). Stainless steel also incorporates about 10% nickel and 20% chromium. Nickel is very expensive, so if you can get away with using something with less nickel, you'd really prefer to! There are "recent" advances in corrosion-resistant steel with smaller amounts of nickel, like "duplex steel" - duplex 2204 is just slightly less corrosion resistant than 316 stainless, but contains 4% nickel instead of 10%.

You only use inconel when you absolutely cannot use anything less. It costs $2.50/lb compared to $0.80 for 316 and $0.40/lb for mild steel (standard run-of-the-mill steel). Not sure what 2204 costs these days, but will be somewhere between 316 and mild steel. No one wants to pay three times more for material if they don't have to, and also pay extra for specialized machinists who are confident working with a somewhat rare material.

Materials scientists are working hard to create new, cheaper alloys with high corrosion resistance, and it drives a lot of really cool new science! Over the past 1-4 years I've seen some super interesting articles here on HN about new science being discovered around this, but I can't find them at the moment.

That said, Inconel is only 4% denser than run-of-the-mill steel, so it's not going to make a difference which of these alloys fall from space onto your house or head.