It is because:
(1) he is in favour of Britain remaining in the EU and has become pro-EU (see here);These things are becoming politically toxic all over Europe, for good reasons.
(2) he is in favour of unending, catastrophic mass immigration into Britain (see here and here).
(3) he is in favour of Turkey joining the EU, a measure which would be a catastrophe for Europe (see here).
Jeremy Corbyn is in favour of catastrophic policies that will continue to cause a massive swing to the populist right all over Europe, and in Britain. Corbyn is more likely to accelerate the collapse of the left through these disastrous policies, and to drive people away from the left.
It is easy to find such people. People who were once left-wing but have been driven right away from the liberal and Labour left and who now despise the left for its extreme political correctness, its hatred of free speech, its toxic identity politics, its pro-EU policies, its betrayal of the working class, and its bizarre obsessive-compulsive mass immigration policies of culturally-incompatible people. E.g., the YouTube personality Pat Condell, as in the video below.
It is people like this, once Labour voters, who are now voting for UKIP.