Because it doesn't delete from the server only the cilent being the email app. The only thing you can do is log in from the browser and clear it that way but the email app in 2.0 works much better but still suffers from the same thing.
Tryed to delete emial but it is still not deleting from my trash
You don't have to go to the browser. I've found that you can go to "trash" and check all the e-mails, then select "delete". I have not found an "empty trash" function for my Prodigy (Yahoo) account. This is a hassle but it works.
It's a known issue. Seen it on Google forums. Android still prefers Gmail (of course). Maybe it'll get better when we get upgraded from 1.6 (hope, hope). About the only fix is to actually delete the email account and re-install it. Not really worth it unless it's really lagging the phone. Just ignore the trash folder.
I'm starting to access my MSN mail through the browser. Nicer UI.
@queueman: I don't see where you can "check" emails in the other email client on the MTG3 in order to delete them. Even if you open each one and use the "delete" button, they still don't go away