The two headphones differ in their acoustic design and thus also in their sound profile.
The DT 700 PRO X has closed-back earcups, which gives excellent sound isolation if you're recording, and you also get a tighter and more focused soundstage.
The DT 900 PRO X have open-back earcups, which open up the soundstage and provide a rich and wide stereo image.
In the Classic DT Pro range, there’s a pair of semi-open back headphones, the DT 880 PRO and these are much-loved for their wide and spacious sound but particularly their excellent treble response.
Due to some very clever design the DT 900 PRO X both the DT 700 Pro X and the DT 900 Pro X feature this same excellent treble response, so there was no need for a semi-open-back version.