In a word, effort.
Creating some kind of contest, curated series of events, themes, challenges or anything which would encourage participation.
Maybe hold a competition for a house interior, for example, over the course of a month or two wherein the winner gets their player house added to a location in the overworld? Or maybe have a special set of 3 or 5 warpers that provide access to the winner's house in game to direct interested players there?
Alternatively, working within the bounds of the systems already in place, have a rating system where you allow an individual to rate a house only once in a month or season. The limitation could garner a degree of concern relating to which house they should rate and why. Rate a house at a high rating in order to boost its visibility? Rate a house low so that it doesn't compete? I would recommend (and have recommended) a rating scale of 1-5 in order to generate an average rating for each house.
Keep some kind of record or competition that is governed by impartial staff members. Have a list of the top 50 for the season culminate in some kind of voting scenario for a 'staff favorite' or 'people's choice' categories.
Holiday themed events for builds or counter-season events (christmas in july, tropical vacation in december, etc) with an entry fee and prize....
Basically involvement and investment of time and effort could generate an entirely different focus when it comes to the building aspect of the game. For the record I've given tons of ideas over the years on this and would be glad to come up with something more elaborate if required.