If a teacher claims enlightenment how can she/he prove it? She cant and for most of the time those listening will think she is a fraud. This will turn them away from listening to her any longer. Alternatively a small number of the congregation might believe her, if only after some explanation- they might become more eager to practice in the way taught, having gained more faith in the teacher. This is a balance every teacher must decide how to strike for the sake of her students.
At the same time true attainments are intensely personal phenomena- often unable to describe using words. I am reminded of a 7 yr old arahanth who according to the buddha apparently though 'may no one know I have attained enlightenment'. So it is not with any ease that these things will be revealed.
Another element of this is that often people are fooled into believing they have attained - and often this happens more than a few time for good meditators in their path upwards. So no one can be 100% certain about any attainments. So not making any prnouncements is often a wise choice. The best way to know about an attainments (I'm talking of magga-phala here) is to complete the next higher one- then one can be certain that the lower one has been reached atleast.
I think another factor is that some teachers have no attainments that they can speak of -so they dont.
Having said all this there are teacher like Ajhan Mun who have spoken openly about the path that they took to instruct their students.