As aluminum does a better job for much less money
Titanium and steel are super strong which means you can use less material for a bike frame. thinner tubes mean more flexy tubes which can be nice on a hardtail making it more complaint.
Aluminium is much lighter but also not as strong so more material is required in a frame. This makes the frame stiffer which isnt always ideal for a hardtail (unless you’re racing) but it is just what you want for a full suss. You’re not interested in vertical flex as the suspension takes care of that and any lateral flex you have will be accentuated by all the pivot points. This will make the bike feel noodley and also but strain on your pivot bearings and bushes making them wear out quicker.
The only way to avoid this is to beef up the tubes – but the material is much denser than aluminium so the bike will become heavy.