The Nexus is Magic
Originally, the studio execs wanted to have both crews involved in the same story, preferably at the same time. The writers couldn't come up with a good idea for doing this (hard to believe for a series that was constantly jumping all over time).
Eventually, they settled for bringing just Kirk and Picard together, but needed a plot device to do it. And yeah, they settled on the Nexus. A little piece of space magic that allowed them to bring two characters together.
Hard to believe that this was the best Ronald Moore could come up with.
"Lovey-dovey. Bonk bonk on the head!"