I'll be honest, I don't really know the reason the polls are out, all I can say is that they just observably are in most instances. Crucially they are out in swing states - Virginia, Minnesota and Michigan have all seen big polling errors - North Carolina bucks the trend. There are plenty of theories out there as to why that is, but it's ultimately all speculation.
@Tyler from Ohio mentioned on the previous page that a lot of these primaries were open - which is true and that will have some impact. But you necessarily have to square that off with the Democrat side of the primary. The confusing bit is you're seeing almost the exact opposite pattern with Biden - he's being underestimated by polling. If there was a really substantial amount of code-switching going on, you would expect to see every vote for Haley is a vote lost for Biden. But this isn't really happening on any substantial or visible scale. In fact the opposite is happening!
I think the reductionist view is that polling in such a polarised environment is fundamentally really hard to do and data gets skewed by the people who are psychologically motivated to be more vocal about their support. We know it as the shy Tory effect. It's the best guess I have.