It appears that Bastard!! was discontinued due to issues with someone in charge of the series, and there was not enough interest or commitment to reviving it later on. According to AnimeNewsNetwork:
Bastard!! was suppose [sic] to be eight episodes, but the creator was arrested during the making of the seventh episodes [sic] and was never continued.
I found this statement mirrored by some fan reviewers and forum threads, but unfortunately it does not seem that there are many reputable sources confirming it. This is partly because I was not able to track down who they referred to as the creator (since they are not referring to Kazushi Hagiwara, the manga's creator), and partly because the internet doesn't always have particularly good information on lesser-known events from the early 1990s.
However, I did find out that screenplay writer Hiroshi Yamaguchi[ja.wikipedia] retired some of his roles, including his role on the production committee, in 1992 (the year the Bastard!! OVAs were produced) and went to work with a smaller group before going independent in 2001. This may have contributed to the cancellation as well.
Lastly, based on reviews and threads talking about the OVAs, it seems they are not as in-depth and were never as popular as the manga. This may have been a contributing factor to the decision not to resume it with a more willing crew.