https://legacy.netdevconf.info/0.1/papers/UDP-Encapsulation-...
В документе подробно описывается, почему выбран именно UDP.

However, encapsulating using UDP
provides some distinct advantages:
• Hardware optimizations for scaling, such as RSS
(Receive Side Scaling) and ECMP (Equal Cost
Multipath) routing, can be leveraged. These can
provide significant performance benefits.
• The UDP checksum provides protection against
packet mis-delivery. This especially relevant if a
packet is being encapsulated in IPv6 which does
not include a header checksum.
• Hardware support for UDP checksum can be
leveraged. NIC support for UDP checksum
offload is ubiquitous and can be used to offload
inner checksum calculation.
• The destination UDP port provides a demux for
different encapsulation methods or encapsulation
protocols.
• UDP allows extensible encapsulation protocols.
For instance, some proposed protocols include
sending optional data with encapsulated packets.