It's more cost-effective to build 2 channels into one case and although the sonic benefits of monos may be limited, they're more versatile and typically built for more power and hence, usually an upgrade.
Why monoblocks?
Why run monoblocks? Most amps can handle most speakers, especially those that are 88DB and above in sensitivity. I see pictures of all these speakers being driven in monblock configuration. If the stereo amp can do the job, why spend money on two amps? I mean, what are the benefits?