I think most people (maybe not you ) try and be optimistic and positive. Boris gave more of that outlook than Corbyn, yes it is true. I also think 'patriotism' comes into it. Boris came out as pro-Britain, pro standing up for Britain - however trite you may view that.I think you're restating that Brexit was still a major factor in the GE and can't discern any difference in your comment about that
Corbyn, although I've always said he had some good ideas, was unpalatable to what is mostly a small "c" society. But I do believe personality also played a significant factor. For example, Brown was probably potentially a better P.M than Blair, but he was a bit arrogant and not user-friendly. Blair won 3 elections and Brown didn't win any. Basically Corbyn came across as a miserable, grumpy sod (like me ) whereas Johnson was jolly BoJo. It matters.
Corbyn came across as pro-anyone-but-Britain. IRA, Russia, Palestinian terrorists - all received support. He was/is lukewarm on the need for our Armed Forces, lukewarm on the Royal Family, lukewarm on the Union, anti-NATO and absolutely anti-Trident. Any of those positions is fine to take and there are millions who will support those positions. The issue is there are many, many more millions who don't.