Why weren't there more crossovers in Trek?

[BridgeConsoleMasher]

It would have been cool to see captain Jellico in DS9.

[Darth Wedgius]

Voyager was displayed in space and TNG and Enterprise in time, to where they only got cameos (after the pilot) in extraordinary circumstances. I guess it was coincidence that Riker was part of the crossover both times.

For TNG and DS9... Well, TNG is mostly about taking the Enterprise to new places, and DS9 wasn't. In practical terms, the Enterprise would be going where plenty have gone before sometimes, too, and "You are Cordially Invited" would have been a nice place to see at least some of TNG's cast. Not that it was unreasonable to have the ship and crew be busy during a war.

[GreyICE]

Crossovers imply a level of continuity that B&B Trek simply didn't have. Even one of the cases that they could have had continuity easily - Tom Paris crossing on to Voyager - they chose not to have continuity deliberately.

Without ongoing plots, crossovers are little more than fanservice anyway.