Evidence. And the severity of the crime, too. No statute of limitations on capital murder in any state, that I'm aware of. Also, let's say someone gets pissed at someone, then 20 years later, that person claims "rape!" just to get revenge for a rape that may OR may not have happened...where is the evidence? Likely 99.9999% of the time there is none. Certainly nothing that would stand up in court, or used by a prosecutor to push a case forward. Plus, like with all crime, the burden of proof is on the prosecution.
Would YOU want to be hauled off to court on a 20-year old traffic ticket? Without a statute of limitations law in effect, that COULD happen! You punched someone in the jaw 15 years ago and were never charged or prosecuted. Cops bust your door down for that and haul you off. You really want that to happen? An old girlfriend or ex-wife accuses you of rape from 2002. Cops haul you off. THAT is why these limitations exist.
Think about it.