Because he was already at the club for a few years. It's not the same seeing a player develop than seeing a player have an immediate impact. And Vinicius was pretty hyped too when he arrived in Madrid, until the excitement kinda died down due to his bad finishing, then Hazard (briefly) becoming the new shiny toy.
People are always excited with new, big signings. People are always excited with young, talented signings. People are always excited when a new signing immediately performs, and even more if it's above expectations.
What we're seeing with Bellingham is the addition of all that.