While it may seem to non-native speakers that using ‘seulement’ might be simpler, the negation ‘ne … que’ is used more frequently by native speakers, in my experience. 

Especially as dropping the ‘ne’ is common in speech, it flows very easily in speech, and the specific placement required for ‘que’, immediately preceding the thing/idea being restricted, makes comprehension quite straightforward. 

Not sure if you can expect a definitive and undisputed ’reason’ as in language usage patterns and grammar, the answer to ‘ why ‘ is very often just ‘ because that is the way it is ‘.