We weren't given the exact reasons behind this quiet withdrawal, but our friends over at MoDaCo might have had the answer for some time -- soon after the release, they discovered that the 2.1 firmware was in fact pre-rooted, thus leaving the Streak vulnerable to unauthorized access and modification (although handy for the seriously tech-minded).
ENGADGET: Dell Streak's pre-rooted Android 2.1 update quietly suspended, revision coming in two or three weeks