I can only guess, but from a coder pespective, TGA is just a super simple, straight forward format. Only an 18 byte header that isnt too complicated 
The same goes for bitmap.
TIFF is a bit more complex, partly due to the multi image support, etc.
http://partners.adobe.com/public/developer/en/tiff/TIFF6.pdf
So TGA is much more convienient to handle 
Also some of the TIFF compression stuff may still be subject to some licensing issues…
PS: BMP is far from being outdated. It has some advantages.