There have been various items put below deck over time - washrooms, galleys, crew quarters, entrances, etc. They all suffer from a range of issues.

Cargo/baggage space may be more needed than the main deck space taken up by washrooms. The A380 has a known issue with this, two whole decks of passengers means lots of baggage underneath and thus restricted cargo space. It's one of the prime reasons driving proposals for an A380 stretch.

Aircraft structural issues divides the lower area into a number of separate compartments which the facility may not fit well into. The wing main spars come through, and there are other structures as well.

Stairs themselves are space-wasting, both in the main and underfloor areas, and there are only a few locations in the airframe where you could put them, maybe not particularly suitable for seating efficiency on the main deck.