NFS used to default to UDP for the same reasons that games still would use UDP, it just needed the data as quickly as possible since it was blocking IO. Now a days, we don't need UDP for NFS because networks are a lot faster and the advantages of TCP start to over rule UDP.

The original reason was that using UDP was much faster CPU-wise back in the 80's. There was a requirement to go fast because it was used for transferring data in bulk, not events like games.

When Linux came around it just followed suit. TCP support came to NFS only later with other bells and whistles in later NFS versions, and Linux stuck with basic NFSv2 for a good while.

NFS was designed for local-area networks, so it didn't need to worry about working over the Internet.

In addition to the network being much faster relative to CPUs back then, the TCP stacks hadn't seen the tuning they have now. Making TCP fast came later.

NFS (was) stateless and idempotent. Lose a packet? Send it again! Which is mostly what you want, since there can be dozens of processes performing IO and they're all independent. Last thing you want is head of line blocking.