Do you mean on the mudcat midi site or in the DT. For the DT you'd have to ask Dick Greenhaus - and I suspect because the DT has always focused on songs.

As for here, see above. The midi's for songs are for the most part a by-product of preparing them for the DT; since the instrumental tunes aren't harvested for the DT. . .

BUT - if they are posted in ABC - you can select the ABC notation, copy it and paste it into the box on this site [http://www.concertina.net/tunes_convert.html] and hear it or produce tadpoles.