Mercury, Venus, Mars, and Jupiter were named after living Olympian gods. Yet Saturn is named after Jupiter's dead father. Why was Saturn named after a titan when all the other known planets were named after Olympians?

Saturn as a Roman god certainly predates importation of the Greek myths. His temple at the base of the Mons Saturnius (later Mons Capitolinus) definitely predates any association with Cronus. In these pre-Greek years he had a number of non-Cronus characteristic including the fact that he was a mixed sky/agricultural deity, his sacred dates (in mid-winter rather than mid-summer), the fact he was living, and the fact he was married to Lua rather than Rhea. Although of course Lua herself is something of an interesting goddess who appears to have eventually been subsumed into Cybele and Ops and eventually picked up some Rhea like qualities.

Even after 217 BCE when Greek rituals were added to the Saturnalia, he wasn't ever completely subsumed into the Greek Cronus. Even once he was attached to the Olympian myths, he remained a unique figure. The Romans adopted a myth that he wasn't killed but instead fled to Italy where he ruled the Italians in a golden period protected in part by Janus and Terminus. He was said to have established his cult on the future site of Rome, founded a number Latin cities, and fathered Picus, the first king of the Latins. Eventually Jupiter bound Saturn, but he's released once a year (during the Saturnalia) to break social bonds and bring upheaval and change to the world. As time passed he had a habit of synchreting other gods so in addition to things like being a god of weather and agriculture, he also became a god of time, seasons, cycles, truth, order and disorder.

His identification with the dead Cronus thus remained problematic throughout the period where he was worshiped. He wasn't the dead and absent father of Zeus. He was a very much alive and very active important god. His feast was one of the most important on the religious calendar. Of course heterodoxy wasn't exactly a major concern for the Romans. If the Greeks though Saturn was Cronus and worshiped him differently (or not at all) wasn't a problem for them.

We don't know when the planet became associated with the deity. But whether it was before or after, Saturn remained one of the major gods of Rome. So it would make sense for the planet to become associated with him at any point.