Most of the custom ROMs out there were based on CM with their spin to it. CM supported more than 500 devices, the only other ROM that is even close to that is

Most of the custom ROMs out there were based on CM with their spin to it. CM supported more than 500 devices, the only other ROM that is even close to that is MIUI which supports about 200 devices (most of which are in China). Other ROMs support less than 50 devices. Some even keep it limited to Nexus devices only (read CopperheadOS). AOKP is based on CM AOSB isn't being officially updated to even Lollipop I think. SlimROM is active but yet again quite limited and slow. They released their ROM based on M late this year which is very late in terms of custom ROMs standard (even most OEMs by then had released M for their devices) Paranoid Android is active but limited to about 30-40 popular devices. CM was known for supporting devices which the OEMs ignore after 2 years of launching them. And once CM was released for it many other custom ROMs were developed based on it to keep devices updated. This is the main reason why CM is most popular and appreciated for.