That's a bit of a fig leaf: dart never had the clout and popularity Python has, and it was clearly something no other browser vendor could get behind. Python has significant cross-vendor mindshare and wouldn't be perceived as fostering anybody's agenda.
It's true though that getting even two major browser vendors to agree on supporting another scripting language in addition to JS, is really really hard. I don't see it happening unless Nadella's "Really Good, Honest! (TM)" Microsoft makes some sort of grand gesture towards Google or (more likely) Mozilla.
Python might have clout, but in many way it's even harder to optimize than JavaScript, whereas Dart was designed by JavaScript VM experts to be easily optimizable and has a very fast, very small VM.
Python is also only just starting to explore optional typing which is obviously a prime feature of Dart and JS derivatives like TypeScript.
It's very clear that this is not a technical problem, but a political one. Dart could be the second coming of Knuth and still nobody but Google would implement it in a major browser.
It's true though that getting even two major browser vendors to agree on supporting another scripting language in addition to JS, is really really hard. I don't see it happening unless Nadella's "Really Good, Honest! (TM)" Microsoft makes some sort of grand gesture towards Google or (more likely) Mozilla.