There is at least one good consequentialist reason for punishingattempted murder less severely than murder. If the two crimes arepunished equally, then the law will not deter someone who has tried andfailed to murder from trying again!
To the extent that the"successful murderer" is simply luckier than the "failed murderer,"your question raises the vexing problem of moral luck. Consider the following two cases:
1. John drives after drinking way too much at lunch and is pulled over almost immediately and arrested.
2.Jack drives after drinking way too much at lunch and almost immediatelyruns over and kills a family of four crossing the street.
Itseems that there is no morally relevant difference between these twocases. That is, the only difference between them seems to be that Johnwas lucky enough to have been pulled over before he could cause anyserious harm. Yet (at least in our current criminal justice system), wepunish Jack much more severely than we do John. John is charged withdriving under the influence, but Jack is guilty of vehicularmanslaughter. Should John receive more lenient treatment merely becausehe was lucky?
Two of the best-known essays on moral luck are by Bernard Williams and Thomas Nagel. You can find them here and here, respectively. There's also a nice (and free) overview in the online Stanford Encyclopedia of Philosophy.