Did you buy half track tapes?
that's two tracks forward, wider tracks, twice the magnetic material for signal than 4 track's narrower tracks. The deck would need to have 1/2 track heads, cannot play 4 track.
15 IPS, is similar to wider tracks, in that the faster speed gives you twice the magnetic material for the signal (length). The machine needs to be designed and maintained to handle the weight of a full 10-1/2" reel of tape, and the precision of all of it's mechanisms to maintain stability at that speed, no easy feat.
I gave away the few 15 IPS machines I had (don't record anymore), and went with 7-1/2 IPS to allow me to buy pre-recorded tapes at reasonable prices.
Yes, 15 IPS is better, however 7-1/2 IPS is good enough to be the best sounding source in my system.
'good enough' is always a compromise, but not having many pre-recorded tapes to listen to is also a compromise.