Imo it's his character and "authenticity". People like him. He "tells it like it is". Handily he also has a pretty wide range of opinions so at some point he's quite likely to have expressed a view people can agree with.
But mainly it's that he's charismatic and people like him.
I think the whole movement around Jacob Rees-Mogg can only be seen as a thoroughly good thing.
But I suppose that's only because I think he's an unelectable, cretinous throwback, and I suppose similar arguments were made about Corbyn.