hmm Theory....is it due to the small population of Tau throughout history? Eldar owned the universe for a bit which is why they are masters at it. Humans took over the galaxy and that's when psykers started showing up (pre-emperor dark ages of technology). Tau have just showed up on the intergalactic stage and so not enough souls in the realm of souls to manifest and make tau psykers.
just a theory
The Tau are 5000 years old, they are a very young race. The Eldar were created as a very strong, psychic race also very long lived and with heightened senses. Humans are also a psychically powerful species, just longer in the making. If everything would work as intended, the whole of humanity would in time become as strong as the Emperor in the field of psychic might. That was his plan initially. Also some of the Inquisitors, mainly from Ordo Hereticus, know this and try to make it happen by keeping humanity going on, despite everything that galaxy's and the Four are throwing at them.