Why was UDP used as the standard protocol in NFS systems up to version 3 and TCP in version 4? Unfortunately, I have not yet found anything about the motives that led to this.
The Red Hat NFS documentation provides some insight about why UDP was preferred:
When using NFSv2 or NFSv3 with UDP, the stateless UDP connection under normal conditions has less Protocol overhead than TCP which can translate into better performance on very clean, non-congested networks.
...and why the move to allow and eventually require TCP was made:
However, because UDP is stateless, if the server goes down unexpectedly, UDP clients continue to saturate the network with requests for the server. For this reason, TCP is the preferred protocol when connecting to an NFS server.
The current RFC also notes that TCP facilitates transport security without needing to tunnel:
Historically, NFSv2 and NFSv3 servers have resided on port 2049. The registered port 2049 [RFC3232] for the NFS protocol SHOULD be the default configuration. Using the registered port for NFS services means the NFS client will not need to use the RPC binding protocols as described in [RFC1833]; this will allow NFS to transit firewalls.
Why TCP specifically? Actually, the RFC says SCTP is also acceptable (though TCP support is mandatory):
Where an NFSv4 implementation supports operation over the IP network protocol, the supported transport layer between NFS and IP MUST be an IETF standardized transport protocol that is specified to avoid network congestion; such transports include TCP and the Stream Control Transmission Protocol (SCTP). To enhance the possibilities for interoperability, an NFSv4 implementation MUST support operation over the TCP transport protocol.
Finally, addressing performance, the original reason UDP was chosen, the RFC says:
If TCP is used as the transport, the client and server SHOULD use persistent connections. This will prevent the weakening of TCP's congestion control via short-lived connections and will improve performance for the Wide Area Network (WAN) environment by eliminating the need for SYN handshakes.
Ultimately, only the engineers at Sun Microsystems (as the creators of NFS) know the exact reasons UDP was chosen and only the IETF working group for NFS (as the maintainers) know why the switch to TCP was made.