They scrapped Batgirl, which had a much smaller budget, for tax reasons. They could only do that for a brief window of time. In addition to Flash being vastly more expensive, it's also supposedly very good. Batgirl was made for a streaming release, too. Movies made for theatrical release often have contracts in place with the talent and producers that guarantee a release. I don't know if that's the case here, but I wouldn't be surprised.