The problem are the network appliances that are sitting between you and the server. i.e. the whole internet. To support feature X, everything between you and the server will need to support that (unless it's backwards compatible, but that's not always the case, as described in the article).
Decade-long adoption will solve this problem, until one day your packet gets routed through some router running Linux v2.5 and your connection silently fails.
This isn't good enough to build a faster (and more reliable) internet on, whereas UDP is a 40-year old standard, and we can assume everybody supports it, even Linux v2.5