I think it's more of a cultural thing. Sure, there's the scientific aspect of boys developing fine motor skills later in life (this comment), but I think at its heart it's just that most boys growing up just want to play physically with their friends. They're not going to keep a journal, and if they do it might unfortunately be something other boys pick on.

Girls can keep journals, read books, draw and scribble without too much attention from their peers. They have more time to cultivate neat handwriting, and more time to actually care about it.

The thing is, my dad's a man's man. When he was younger, though, his handwriting was loopy and cursive and honestly just so pretty. It's not like he's changed in temperament that much - he was never 'girly'. He just doesn't care what others think anymore. He's too old to care, he's seen it all and then some, and now the only thing that matters to him is whether he himself can read his own writing.