Just speculation here, but maybe because the systems are not capable of showing this information (if the FF system and flight manifest are separated, it may be difficult).
With that said, a couple of quick questions:
1. What is the benefit of having the information in the FA's hands anyway? The way I see it is if I fly DL (which I never do), pay for a international biz class seat and receive less attention than someone else on the plane (who may or may not be diamond elite - I will never know), then I might be upset enough to never fly them again. Ask an Asian female if they like flying SQ, and you will get the same response (from what I have heard - use this as an example, and please don't blast me). I am not shooting you down, just don't understand the benefit of having the FA's with this sort of information.
2. I have been thinking about this for a while - what is the value of a customer to UA? If we fly UA despite better service on international carriers, or whatever, then there is no incremental value to UA by spending more on us. Further, with the amount of switching that goes on, the retention rate is low - this means that the value of a customer is probably relatively small, in their eyes. To increase the value, they may believe they are better off increasing fees ($50 per flight, say), versus having us not switch (which we will switch anyway).
anyway, my 2 cents.