I think you're under the impression that convictions and death sentences are handed out like free samples.

Very few crimes qualify for the death sentence, the instances of those crimes have to be pretty heinous, and even those crimes can be plead down as to avoid the possibility of a death sentence. Even if you manage to overcome all those hurdles, you still need a jury to vote on the death sentence. And then there's the appeals, that could push the date of the execution years past the original date.