The length of the addresses and the clunky nature of their ASCII representation is absolutely the #1 reason the IPv6 has taken this long. User experience is the most powerful force affecting large scale adoption, and IPv6 has poor UX.
I think the UX is partly fixable by creating less horrible ASCII representation, but this would take a lot of coordination that was hard even back then and is virtually impossible now. If someone told me in 500 years we're still running dual-stack IPv4/IPv6 absolutely unchanged, I'd believe it.