we have to say season ticket. We cannot say seasonal ticket.
afraid that it's largely a matter of learning which is the right
collocation. The only advice I can give is the following:
noun + noun structure, in which the first noun modifies or
describes the second noun, is commonly used for expressions which
specify certain kinds of thing.
describe things that belong to well-known classes. Thus we have,
e.g. a return ticket, a season ticket, a history/geography/maths
book, science fiction, a gold/silver/bronze medal,
theatre tickets, milk chocolate.