Because you have to battle Giovanni (rocket leader/vermilion city gym leader)
here i have a little tip for you he uses ground Pokemon he has a nidoking or nidoqueen
sorry i forgot wich one it was and he also has a rhyhorn and kangaskan use water Pokemon or grass
To go back to Vermilion City in PokŽmon Global Revolution you need to go into and out of a few doors. Once you have visited a few doors, you will be able to go back.
go to vermilion city and fly to pewter city
You have to take it with you.
Go to vermilion city and go to the harbor show the sailor your tri pass to go to islands 1-3.
Use the ship that lead you to one island to go back to vermilion city then from there you can use Fly to reach celadon city.
you either use cut just below pewter city (you get cut from the captain of the S.S.Anne in vermilion city) or you go through the other end of the cave on the right side edge of vermilion city (the end of the cave in vermilion city is unprotected so you can go strait in).
Go though the underground path in celebruen city.
Go to vermilion city and go to the dock where you entered the st. anne.
If you are on the Sevii Islands, go to a boating dock, talk to the man by the boat, and select Vermilion City to get back to the original map.
go south of cerulean city
in vermilion city
you have to go back to olivine city and take the boat to vermilion city.