Variety of reasons.
>Early DigitalSonic X, Kirby, Mega Man, these can't be in HD without upscaling, and most DVD releases of older shows like that tend to be initial rental/volume DVDs in Japan with no overall BOX (the ones I listed are exceptions if I recall, but a show like Demikids is not), so that doubles the issue since most people won't buy a dozen individual DVDs at $40+ per DVD just to get slightly higher quality video than the older broadcast rips that tend to exist online.
>OVAsA lot of game adaptations were OVAs like Ys or Megami Tensei (technically a novel but who cares), and these were typically made for LD. I can't think of many video game anime made for OVAs at the time of DVDs.
The next point combines with OVAs.
>Broadcast TV Preservation/Rights IssuesGenerally these have a bit of a mess when it comes to producers since a larger company is being subcontracted by a software company or whatever, so it becomes a mess for the original game company to actually pull this stuff from archival (if it still exists) and remaster it. Usually the original film is just outright lost or the game company doesn't want to commit to a novelty (the case with the Sonic OVA if I recall).
BUT, the irony here is that the anime in the OP did get a full remaster. It's pretty gorgeous too, albeit expensive if you want it legally.
https://nyaa.si/view/2041456It also had a remake not long after since Square Enix was interested in reviving the DQ Dai brand.