If I'm in the middle of the woods somewhere, I can't make a call to
anywhere on my cellphone because it relies on proximity to a network of communication equipment exterior to my device. Communicators in Trek (or real-world tactical communications devices, for that matter) don't have that limitation, and that's obviously a pretty huge difference.
As far as "more capable" goes, real militaries use radio devices that are made for communication, and communication only. Imagine if
this thing was shrunk down to a palm-size device that could also talk to a spaceship tens of thousands of miles up in orbit (without needing its signal to be bounced around a network of antennas and satellites). That would be
way beyond the capability of some smartphone I can pick up at the Verizon kiosk at the local mall.
If I was on a mission on some desolate and primitive planet, surrounded by hostile aliens, I would rather have a robust, single-purpose, dedicated and reliable communications device instead of having to wait until I had two signal bars showing, or having to wait for Words with Friends or Facebook or MSQRD (or whatever other app is popular this week) to un-freeze just so I can make a basic voice call to my ship.
Kor