I do feel compelled to toss this out.
Crossover episodes while always extremely cool do tend to be endlessly difficult if two series are running at the same time due to SGA guidelines.
I'm no SGA expert, but I know there are a ton of provisions in SGA contracts that asks for some absorbent amount of money for one actor in a series to appear in multiple series airing at the same time. It's not as easy as "hire them as a guest star for this episode" or "You're working 40 hours this week, but you're spending 10 of those hours on the TNG set". SGA does a lot of great things for actors, but it makes crossovers needlessly complicated and expensive :-\. It's simply not practical to have an entire cast from one series show up in another series.
That being said, even with this explanation in full view, considering Picard commanded the 7th Fleet; as much as they are talked about through DS9, one would have expected the Enterprise would have shown up more often then it did, especially for the series finale!