Trump and Corbyn both face the same problem in that, while they might well appeal to many supporters of their own parties, they're unlikely to win over many supporters of other parties.
i.e. Even if Jeremy Corbyn gets everyone who voted Labour in the last general election to back him in the next one, Labour still won't win unless he can get a significant number of former Conservative/Lib-Dem/UKIP/SNP voters to switch allegiance. That seems extremely unlikely to happen, as there's little in what he has to offer that's likely to appeal to any of them.
Similarly, if Trump gets everyone who voted Republican in the last Presidential election to back him in the next one, he still won't win unless he can also get the support of a significant number of former Democrats. Again, that would seem to be extremely unlikely.