Because realistically comodo AV is definitely NOT the best AV there is. But I believe that it is the best full security suite that there is.
So my experience with comodo is with the internet suite, it includes the HIPS and firewall, plus my browsing practices are pretty safe. Therefore to me the most important thing is prevention, and comodo is great at doing that.
Also for more experienced users comodo allows a greater degree of customization which is generally not found in free suites.
But if you are looking at AV specifically there have been many tests which show bitdefender to be the best there is. On windows there's also 360safe...it uses 3 AV engines, bitdefender included, and it provides an even greater degree of security, but it is not a suite, and personally I prefer suites.
On linux I am currently using comodo AV, but I would happily switch back to bitdefender if they started developing again their linux software.