I think one of the main reason Flow's progress was stopped was because it lacked the ability to switch back and forth between visual and source editing. It's hard to recreate immediately visually all the features you have while source editing which nonetheless can be achieved with optimization through time. But editors are usually impatient and wikis, especially EnWiki, can sometimes be pretty dynamic environments.
Tangent to this subject I can say that sometimes I feel like EnWiki does get a tiny bit more weight than it normally should in many aspects. Don't get me wrong, as the wiki with the largest numbers of editors it should get "the biggest slice", sometimes even that "slice" may not be enough for the large resources it needs to exist. But some other times the discussions start to go in subjectivity territory and EnWiki's opinions get to dictate indirectly the course of progress for many projects. Flow aside, the whole promising progress with WikiData seemed to come to a halt after the so called Infobox Wars in EnWiki and WikiData turned from a "new project that will revolutionize information gathering and displaying" to "a project that offers easy data for robots and search engines", at least in the eyes of "the general public". Content Translation Tool was at one point at the risk of taking such a similar turn as a project when EnWiki deemed it to be too risky for their standards and I do feel like part of the reason why the Review extension eventually became orphaned was because it was one of the extensions that aren't used in EnWiki. Very generally said, EnWiki likes to do things as organically as they can and that's not bad at all per se. They can afford that because of the enormous resources that they have, in technology and in working power. The problem starts when they indirectly start "vetoing" other projects which can be beneficial in other environments. Of course it's not their aim to do so but it happens by the "peer pressure" they project as the biggest community and most successful community. After all, "it's better to invest at something that it's known to work than to spend time and resources for new unreliable projects".