I think it is because LotR has so many layers that are only hinted at in The Hobbit. The Hobbit is a great book, but it leaves you wondering about so many things that are answered in LotR. I also think that LotR can appeal to a wider audience. The Silmarillion and the Unfinished Tales and the other books are more scholarly and hard to read. The Hobbit, while also being a wonderful book, is written for children, so can only go so far with grownups. LotR can appeal to grownups and children, casual readers and Tolkien devotees.
I love The Hobbit too Celtavhc and don't particularly like one over the other. But that is why I think LotR is more famous.