Hello Dave
It's been a while now, but the 2 main things as far as I remember were the problems with conversion of the Aperture library (as mentioned above), and I didn't like the workflow. There was probably a bit of subconscious prejudice too as I used Photoshop Express some years ago because I needed it to edit illustrations for a text book - and it nearly drove me crazy!
I'd prefer to be positive and reiterate that the main reason was a higher degree of comfort with Capture One than any of the others
. These things tend to be somewhat subjective mind you - one of the first places I go each time I log on to CIC is the current batch of mini comps and about half the time I end up shaking my head in disbelief at what garners (or does not garner) votes!