I guess it's because the 880 is considered the most "hi-fi" out of the 770, 880, and 990. The 250Ω Pro version might give a taste of what the DT880 600Ω sounds like, but at a lower cost. Well, that's my reason for wanting it anyway.
But DT990s are great headphones too. They have the same drivers as the DT880, and the difference in sound is purely from the housing and padding inside. FWIR they have better bass response than the 880. I already have good bass headphones though.