I take it you mean why can't teeps join the Anla-Shok, and what the risk is.
Just my two-penneth, but being a Teep is a double edged sword. You have all these extra powers, some of them truly amazing, but also can leave a trail behind you, and the stronger teep you are, the stronger the trail can be. Of course, there are ways that a teep can attempt to hide this telepath trail that follows them around. For a Ranger, I's guess that is not something that you really want to happen.
Plus, in the show, IIRC there were never any Rangers who were telepaths, and so you could say the precedent is set there. Durlan, from 'To Live and Die in Starlight' doesn't really count, as he had unusual abilities, not straight psi-powers. There was one exception from the B5 CCG - Movekk, who was apparently also a member of the Warrior Caste. Try and work that one out - I've got my own idea, but it's a little bit convaluted - not unexpected really.