Because IQueryable's don't always translate to working SQL. I've broken EF's IQueryable a few times in the past; NHibernate's one was really bad if I recall. The expression inside the IQueryable operator is checked for type safety but the ability to generate SQL from it isn't which happens at runtime when the LINQ is evaluated. The F# approach bypasses the expression -> SQL translation and directly checks the SQL at compile time if it valid against a database schema rather than the expression AST that C# LINQ does.
