By 1995, serialized shows were not the norm. X-Files was the only real one and it was a trailblazer doing something unexpected. B5 wasn't well known and wasn't getting good enough viewers to make Paramount happy if VOY tried the same.
If they had done what Berman had wanted and waited until DS9 was finished then there'd be other serial successes that Berman could show to Paramount as examples of what could be done.
Also, it would've given them enough time to hire a permanent writing staff and iron out the shows' conceptual problems.