Because most protocols can handle message loss, with retransmit and proper ordering ? And we haven't started to talk about congestion yet… TCP is useful, and while I'd like to see a message get rid of one (or more) of those constraints to go with a a custom protocol, I feel like they'd be re-implementing the features in the end because these are very useful properties to have…

Edit: the proposal in the article is actually quite, sensible, but requires redesigning your apps… And I'd like to see how it performs: TCP is a hugely optimized beast (when it works well), with hardware offloads, kernel optimizations, etc.