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Because of the myriad of firewalls and routers that don't understand IP protocols that isn't TCP or UDP. IPsec tunnels were commonly encapsulated in UDP packets for this very reason because its safer to assume that at least one device alobg the way won't understand protocol 50.

"How Hard Can It Be? Designing and Implementing a Deployable Multipath TCP"

https://www.usenix.org/system/files/conference/nsdi12/nsdi12...

Multipath tcp isn't really a new protocol as much as it is an extension to tcp and wouldn't offer the benefits that UDP would as it is still constrained by (and benefiting from) the design goals of tcp.