It's a protection mechanism to prevent people to delete too much of their contribution on the site. It also hinders an hacker to do the same. Actually your action triggered an automatic moderator attention flag by the system, which I cleared just now.
I guess from time to time some people, mainly from the bigger stackexchange sites, may become frustrated for some reason and start to delete their posts and comments. Later they might regret that. It also hurts the site if some more important comments are gone. So I'm not surprised the SX team put them protection mechanism in place. Normally you don't need to delete too many of your comments in one go. You could also flag some as obsolete, so that a moderator can delete them for you. However, I wouldn't do that because its an extra workload for the moderators.