Volume. Online retailers have taken over the trade once filled by local shops. They have warehouses of inventory that they ship all over the world. This gives them leverage when negotiating prices with manufacturers. I suspect that a number of online hobby sites are probably owned by holding companies that own many other sites giving them additional buying power.
That guy at the show was probably sitting on that kit for years. You just happen to be the lucky guy walking by at the moment he got sick of looking at it.
I'm rarely "that" guy.