I'm not sure about this but perhaps the 4th couldn't drag the Kyuubi to the underworld because the tailed beasts are somewhat forces of nature that simply appear in the Naruto-Universe. So it's not like the Kyuubi is the spirit of a certain dead fox that can be taken to the underworld like a human spirit but rather a nature spirit that can't be killed or get rid of (sry for the crappy comparision). Therfore sealing may be the best way to deal with them