It's a shame to see the service go, but I can't say I'm surprised.
When it was introduced the SpamCop email service was cutting edge for its time, offering extremely reliable spam filtering at a time when most other email services were capable of no more than a token effort. With the ability to utilize RBLs and even select which RBLs to use, and later features like greylisting, it was far more effective of a server side solution than anything else. Heck, some spammers wouldn't even hit spamcop addresses due to the fact that it just increased their odds of being quickly reported and added to the SpamCop RBL.
However it's generally outgrown its usefulness, which is reflected in the fact that the service has so few users and now is shutting down. Most email services are utilizing RBLs these days in some form - if only through SpamAssassin - and the largest services such as Google and Hotmail see so much email that they are second-to-none in their ability to identify spam based on heuristics alone. This means the SpamCop email service no longer has the large advantage in spam prevention it once held, and in some ways it may as well be worse since it can't rival Google's heuristics.
Plus the service has generally grown stale. The Horde webmail interface is functional, but badly out of date and lacking the functionality of Google & co's webmail interfaces. And the service itself has grown into disrepair; there have been repeated hardware failures and CESmail (the company that actually provides the service) has been slow in repairing them and responding to user support tickets.
Anyhow, the SpamCop email service lived a good life, but as is the case for many Internet services it has failed to adapt with the times and is now justifiably on its deathbed. The good news is that the SpamCop RBL itself is unaffected (it has been owned and operated by Cisco for several years now), so naming confusion aside the all-important RBL will continue offering spam protection for users world-wide.