As NFS started being used over WANs, it evolved towards also being run over TCP, certainly a win on congested WANs, and fitting better with NFSv3's potentially very big reads and writes. Paul Lin discussed SS7. Several signaling exchanges occur between MGCs in network to set up a call. Scott Petrack discussed IP Telephony. PDUs come in small bursts at call setup and teardown, which makes RTTs added by TCP connection setup expensive.
Reliability is the key.
Timing requirements make retransmission back-off problematic, as it can trigger PSTN timers. BGP peers exchange large messages, requiring some form of segmentation. Because BGP maintains synchronization from startup through shutdown, and long-term associations, it's natural to introduce connections as opposed to datagrams. The utility function is smooth with bandwidth degradation, so TCP congestion semantics are okay. Updates are self-describing, so no sequential ordering or duplicate detection is needed.