Originally Posted by orbitmic

Because there are plenty of cases where airlines will consider you have an acceptable excuse to have missed your flight without it being « their » responsibility to have made you make it.

if you missed your connection because you lost your way in the airport (eg went to the wrong pier), airlines will typically accommodate you on the next available flight, but if that means a downgrade, you won’t be eligible to compensation. Similarly, where airlines have a « flat tire rule », it does not entail compensation if you have to be reaccommodated in a lower class.

in short:

1) you miss your flight because of the airline (eg delay, cancellation), they will protect your itinerary and pay compensation and/or difference if this cannot be in the same conditions as originally booked;

2) you miss your flight due to a cause which is not under the airline’s control but which they consider an acceptable excuse (either formally or informally), they will usually accommodate you the best they can but this won’t give you a right to compensation if this involves a downgrade;

3) you miss your flight due to a cause the airline deems unacceptable, you can try to plead your case but may lose everything.

this is straightforward case no2.