> Then why has the site become more annoying and less usable over time?
I can't and won't speak on behalf of AirBnB, but you should keep in mind that in general:
a) all software grows by accretion until a breaking point,
b) paying off technical debt without solving any concrete problem or adding any tangible benefit is not considered a worthwhile investment,
c) a site did not changed for the worse if the data shows its conversion rate increased.