San Gottardo there is one possible reason that is often forgotten but is rather frequent at the present time: Change of aircraft!
How can you check-in for a flight when the seat map is still unknown?

I have only once experienced this problem with OLCI and it was for the second flight of my journey that I could not check-in for in spite of it being within the 60 hour limit. When check-in finally opened for that flight there was an aircraft change from B787 to A332! Not all of these delays to OLCI start result in an aircraft change but that is entirely due to the skill of the schedulers that allocate aircraft to flights.

Which would you prefer:
To check-in on time using an old seat map only to find out at the airport that the aircraft has changed and you had a new seat allocated
OR
wait a few hours and be able to use the correct seat map?