One thing I just realised about that story is that despite Brendan stealing most of the Ghostbusters' cool moments, in the story it's still Egon who thinks of crossing the streams. I have several guesses as to why that might be:

[*] Rome doesn't like Egon, so he doesn't want much of him in the stories and doesn't want Brendan to take Egon's role This would also explain why he had Ray ask for a sample of ectoplasm in the S(l)edgewick instead of Egon. However, I doubt that's it because A.) he was perfectly willing to steal Egon's thunder in the sequel, B.) If he didn't like Egon, I doubt he'd be trying to ship him with Janine, and C.) I don't think anyone can dislike Egon and still consider themselves a Ghostbusters fan.

[*] Rome feels Egon isn't cool enough for him to rip off Because Egon is stoic and intellectual. Though if that were the case, why would he be willing to steal Egon's thunder in the Janine, You've Changed ripoff? I mean, you could say it's because in that episode, Egon isn't being as cerebral as he usually is, considering the solution was love and not his usual fare of technology and logic, but stil...

And I really hope it's this last one, but that might be wishful thinking:

[*] Even Rome realised that his Gary Stu was too dumb to come up with the save-the-day plan