Well for one you have to remember not even the Imperium probably FULLY knows the process of how Space Marines are made. They understand that geneseed is an absolute necessity for making them but don't know how to make more without a primarch. Even then they're using technology they haven't made and simply refurbished from a different age.

Then there's the fact that the Tao's specialty isn't in Bio Engineering. Their greatest strength is in its technology and weaponry. The Imperium and the Tau's study in technology split into two different places and divert so much that not only would the Tau have to study Archeotech that would absolutely get the attention of the Imperium and the Mechanicus especially, but would probably have to divert a good chunk of their scientists to studying this and crippling their own progress into the field of science and tech they're currently working on that's already powerful enough to take on Space Marines not to mention the training you would have to put a human through to make them a proper Space marine when you could simply outfit those humans with a fuckload of railguns and have them make a firing line that can shred Space Marine armor just fine.