It mostly comes down to money, American Witnesses make less on average than other Christians, Pew published a study a while back showing that, and so a congregation can't afford to make their Kingdom Halls ornate; and like Bad Wolf said what money Witnesses do offer up goes right to the top. The Governing Body even demanded halls empty out their bank accounts except 5k and send the remainder to help with 'building kingdom hall's (i.e., to Warwick) back in what, 2016? 5k doesn't leave much wiggle room for pillars or painting if you need to fix up the roof or replace some windows.
That being said where I live I've seen some nice Kingdom Halls, nothing like a basilica or synagogue, but gardens grow around the property, they put ottomans in the foyer, they frame the TVs in the sanctuary with wood paneling, the carpet's sleek and colorful, those kind of little touches that go a long way. Credit where credit's due.