Johnson is not racist, he has made some clumsy gaffs (as ever) but there has been some remorse to those gaffs. Whereas Corbyn has made little attempt to apologise for anti-semitism in his party, or act on it and is the only other party, other than the BNP, to be investigated for such behaviour. He has called Hamas and Hezbollah “friends” - something that he has apologised for recently (there are some things that you knowingly do, with intent, that an apology just will not do). His latest ‘neutral’ stance on Brexit is another shambolic way of trying to win votes rather than lead for a cause. His stance on NATO and the IND is equally wishy-washy (for his own means too). It just shows that you cannot trust Corbyn whereas you might be able to trust Johnson. So the choice is obvious to many.
PS. Then there is this bombshell in the news today (good timing): General election 2019: Chief Rabbi attacks Labour anti-Semitism record
https://www.bbc.co.uk/news/election-2019-50552068