There's a lot more things on tng-era trek sets that make the physical building of it more of a struggle. If you look at any of the TOS sets, you can almost build the sets in your head, then walk into a room full of 8x4 and build a corridor. The bridge sets are symmetrical, even the turbo lifts are flat panels.
ST1 onwards have a lot more curves, odd angles, cut holes, back-lit sections, etc. Don't even get me started on working a helm console! Unless you're quite skilled at woodwork your looking at money to get it done.
I'm not saying that building a set is easy by any means, just that comparatively STI to TNG and further is more technically complicated.
Skills or money, it's a toss up.
Based on that, I can understand why there's not many tng sets around.