I once strung a dreadnaught with nylon strungs, just because I happened to have four dreadnaughts at the time and someone gave me a set of ball-end nylon strings. It sounded like crap.

Flat-top guitar tops are "activated" by the upward tension placed upon them by the strings literally trying to pull the top of the guitar off. Steel-string guitars are built so that the top achieves its maximum sound potential under the pull of a set of steel strings. Nylon strings exert much less pull upon the top, making it sound "dead". Guitars intended to be played with nylon strings are built with much lighter bracing so their tops respond under less tension.