One of the biggest guest complaints about the Airbnb system is unresponsive hosts, so Airbnb tried to improve things by requiring that hosts affirmatively Accept or Decline a reservation within 24 hours. It's not perfect, but it's the best solution they have right now.
Guests can only send 1 reservation request at a time* - their funds are tied up immediately & they need to hear back from the host before moving on to another listing. You can imagine how frustrated a guest would feel if they got no response from a couple of reservation requests, each taking 24 hours to time out. And there are MANY hosts who are absolute trash at giving a response, so Air blocks their calendars when a request goes 24 hours without an accept/decline .
A few hours in advance of the deadline I'll message "Airbnb asks that I give a definite reply to your request within 24 hours. If you aren't able to reply by 8pm I'll need to decline, but feel free to send through another reservation request if you'd like to continue the conversation". In my experience, the ones that send a reservation request, but can't respond to questions within 24 hours aren't the greatest guests (barring internet/cell accessibility issues), so I don't sweat it too much.
*not to be confused with an inquiry - which they can send out as many as they like