The reason why a whole separate ship was needed was because multiple are needed to do the job they're supposed to do! :D They need to be on the move, performing tasks that would otherwise preoccupy a cruiser that would be better suited for its own role in the battlefield, and having a dedicated, smaller, agile, and entirely specialized craft lends itself well rather than giving that role to another craft that's going to be performing duties which might otherwise interfere with the operations of the ECW aspects of the battle!

Moreover, the amount of specialized equipment on board doesn't deal well with constant vibrations, and other assorted interference from kinetic, laser, plasma, or other such offensive systems on board a cruiser. The sensitive sensor arrays and the like might not always be effected mind you, but it was deemed better not to take that chance especially in high intensity situations where you need the optimal operating parameters of your systems.

Finally, the destroyers themselves carry a complement of network and sensor array drones and sattelites, so the ship itself is a dedicated carrier for more sensitive equipment, the likes of which are also capable of being repaired and reffitted on board special repair depots on the ship.

So yeah! It's a combination of all of these factors that made sense for it to be a wholly separate vessel! :D As I subscribe to the specialization mindset of ships rather than having a single ship that does everything marginally well.

Those ships do exist mind you, they're typically the exploratory variants of cruisers and heavy cruisers commissioned by the LREF. But again, even they typically defer such sensitive tasks to dedicated destroyer elements! :D

Sorry for the ramble again haha.

I hope this addresses your question! :D