Because it has to be made certain that Jesus was really dead. If he was resurrected few hours later, modern Bible readers would think that he was just in a coma and he was not really dead.
The three day period is one good way to make sure, beyond a shadow of doubt, that Jesus did die and he rose again. The truthfulness of the Christian religion rests mainly on the resurrection. If Jesus didn’t die, he could not be resurrected. And if he was not resurrected, our faith is useless (1 Corinthians 15:14).