Why isn't the inline design suitable for 8-cylinder engines?

While there are some 8 cylinder engines that have been made in the past that are inline, it is mostly prohibitave because of the length of the engine. Its too long to fit in most cars. Hence the V8, or straight 6.

Also the straight 8 motor design is a balanced design that does not create bad harmonics at high rpms, and has what we call "inherent primary, and secondary balance"

Some examples of a straight 8 are 1940's Oldsmobile engines, some prewar packards, airships, airplanes, boats. All sorts of things, but they were phased out after WWII for vehicles mostly.