It's partially logistical, true. But, what also feeds into that is the AdMech, and the general loss of knowledge that allowed it to flourish. Yeah, there are many individual things in the Imperium that beat the Tau. The problem is that it's largely individual things which can't be reproduced, or if they can must be hand crafted. Artisan works are all well and good, but it's shown that they can barely supply the Adept Astares with such works.

The Tau presumably will not have this problem, at least not until a Dark Age or something.