It's their forbidden status and coastal proximity that makes them more interesting, but if they were down in Peru or Bolivia they'd be outliers nobody cared about
For me at least, the thing that makes the Colombian Andes (and the Sierra Santa Marta) the so interesting is that you start climbing them in the tropical rainforest and end up in the glaciers. You pass through many vegetation and life zones since the major ranges of Colombian Andes start near sea level instead of from a high plateau like you do in Peru and Bolivia. There are interesting archaeological sites around the Colombian Andes as well (though Peru and Bolivia have interesting sites as well). In the vicinity of the Sierra Santa Marta is also the Ciudad Pierda/Lost City, site that I think would be awesome to go to.
As far as big, rugged, glaciated mountains go, Peru and Bolivia are better. As far as the change in vegetation, fauna, life zones, and changes in scenery in a short distance, few places in the world can match the Colombian Andes. The only places I can think of that might match (or possibly exceed) in this manner are perhaps the highest peaks in New Guinea and the Rwensori.
The Colombian Andes are beautiful and special in their own way. I like them as much as the Peruvian and Bolivian Andes, but for different reasons.
These old photos aren't the best, but I took them on the rainforest approach to Nevado Tolima (the photos don't do the place justice though):
You start the trek in jungle type scenery with rainforest, waterfalls, monkeys, and even some hot springs. A few days later you are up in the glaciers:
I find the high mountains of Colombia to be very attractive, but for different reasons that I find the high mountains in Peru and Bolivia attractive.
Variety is the spice of life and mountaineering. There is nowhere else in the world I have been that is quite like the Andes in Colombia. I would go back in a heartbeat.