There are a lot of reasons for this. First of all, Tolkien wasn’t a full time writer. He never left his job as a philology professor, where he was considered to be very influential and one of the leading experts in medieval English and Norse literature. The Lord of the Rings took him a lot of time of edition and revising, he probably wouldn’t have published it at all (nor The Hobbit) if his friends wouldn’t have encouraged him to do so.

Secondly, it was his editor that wanted more books and stories, after the commercial success that was LOTR. However, Tolkien didn’t want to write something new, but rather revisit old tales and poems that he had worked on while he was a student. These were very incomplete drafts of The Akallabeth, Beren and Luthien, Children of Hurin, The Fall of Gondolin and so on. Some of these were written (and never completed) in a way that imitated the style of the medieval poems. His work, then, wasn’t easy: he had to transform these drafts into something that could make narrative sense, and make all these stories work together and be consistent to each other, when they were never intended to do so (there was, obviously, a common theme: the First Age of the Middle Earth, the ruin of the Silmarils and the fall of the Elven Kingdoms).

And finally, Tolkien was getting old. By the time he published LOTR, he was 62. The book brought him fame and fortune, but at the same time, disrupted his family life and he became increasingly annoyed by fans and the fact that he was now a renowned personality. This greatly affected him, and after retirement, he moved to a more quiet place. He missed his friends and he wasn’t motivated in the same way he had been when he was writing LOTR. His work rate kept decreasing and after the death of his wife in 1971, it is safe to assume that he didn’t make any advances to the books. He was almost 80 by this time and died shortly after.

Source: JRR Tolkien: a biography, by David Grotta.

Gotta love when the comments are shadow banned.