The upcoming Avengers movie follows on from several other movies based on the Marvel universe: Iron Man 1 & 2, Thor, Captain America and the Hulk reboot.
If I'm not mistaken Black Widow appears in Iron Man 2, and Hawkeye makes a bit-part appearance in Thor.
Is there any reason they were not featured in their own movies?
I'm only concerned with an in-universe reason possibly linked back to their role in the Avengers comics, not an out-of-universe reason (which might be subjective and devolve into 'Why did director X do Y, that sucks?' as warned against in the faq).
Edit: the format of most super hero movies is action-packed, excitement-filled stories. If Black Widow and Hawkeye's in-universe role in the Avengers is not conducive to a good film premise it may not have been suggested and/or accepted.