I don’t want to get into an ideological flamewar (hello dang!) but this factually isn’t true. Momentum have gotten so out of hand that even Tom Watson is telling people to vote for BoJo now.
Momentum aren't even in the same league. They are far less influential than Militant were, or the ERG within the Tories. The majority of the MPs aren't supporters. Tom Watson more likely takes issue with Corbyn's impossible dance with Brexit, and the clique around Corbyn - at heart Corbyn wants to leave too, or the failed attempt to oust him.
Momentum appears to be a mostly failed attempt to recreate those Militant days, except they are a far less successful parasite.
https://www.publicfinance.co.uk/feature/2018/10/liverpool-19...
Anyway I am just making observations here, not trying to start an argument :-)
The right seem to have had their Internet act rather more together - whether the ERG wing of the Tories, UKIP, or the outright fascist Britain First. Or at least slightly less inclined to split into the People's Front of Judea, Judean People's Front, Popular Judean People's Front - membership of one, etc.
Maybe a better question is why the far left haven't been able to replicate what might seem pretty obvious for a fringe group? I suspect in other parts of Europe they are doing much better at it...