Because nothing inside the file identifies the contents of it. The '8' indicates that it is a bank-switched cartridge ROM in non-inverted format.
If you got the file raw without any documentation, the 8 tells you everything you need to know to load it. No identifier, and you start guessing.
Thank you Tursi!
(it will work with other emulators as well of course, but love to do this with Classic99)
Star Trek "Duos" - Never play home alone version. More fun with the family!