For intercontinental flights I am advised to be at the airport two hours before the gate closes. For intracontinental flights one hour is enough. Why is there a difference?
The rule is actually 60 min for domestic, 120 min for international. Some companies, like Delta, even require 3 hours for international flights.
That could have a series of motivations, mainly due to convenience.
Bigger planes imply more luggage to load, more people to wait, more people to get on board. It seems reasonable that they don't want the plane to get delayed with people who came barely earlier than 30 min. And they obviously need to admin on board everyone who came before the stipulated time.
Needs of a domestic flight:
- check in
- check luggage
- go through security
- board plane
Additional needs of an international flight:
- valid passport check at check-in
- visa check at check-in to ensure you have the rights to enter the destination country
- (not every country) exit passport control - outbound stamp, check
- (not every country) departure forms need to be completed, checked
- (cynically, you could argue extra time allows more spending at duty free)
While not always the case, international flights are more likely to have more people, more luggage to load, food to load, and additional paperwork, refueling and checks. Of course, most of that can be done while you're processing land-side.
Several things play a role. Others have mentioned passport control and immigration but this shouldn't be overstated. US-based passengers might mistakenly assume that the only thing that matters is whether the flight is domestic or international but it's just one factor among others. In Europe, it's not uncommon for non-EU non-Schengen flights to have short recommended check-in times even though passengers need to complete all the usual formalities and the distinction is really between intercontinental/long-haul flights on the one hand and shorter flights on the other hand. The layout of the airport, terminal used, size of the aircraft (and consequently the number of passengers and amount of luggage), etc. presumably all play a role.
Therefore, it's not uncommon to have different recommendations for each airport, specific requirements for sensitive flights or different recommendations for different airlines at the same airport (I have occasionally noticed shorter minimum check-in and boarding time for local airlines). From Europe, US-bound flights in particular seem to have a longer lead-in time than other comparable destinations (possibly because the airline needs to communicate the passenger manifest to the US authorities?).
Technically, you can also find flights that are intercontinental, yet domestic, e.g. flights between mainland France and Martinique, Polynesia or la Réunion. They typically use large jets like the Boeing 747 and follow the same check-in rules than transatlantic flights and other long-haul routes (incidentally, there is a passport check but on arrival).
Many countries in Europe don't even have any domestic flights at all, and even in countries that do most people don't have a concept of "domestic" versus "international" flights like the US for example does. Nowadays with most countries in Schengen, the EU or both, flights within Europe however have the character of domestic flights like in the US.
So the main distinction, as it exists in Europe is "continental" versus "intercontinental". And since the major intercontinental destination is the US the rules for intercontinental flights are adapted to the requirements of the US.
Unforseen delays can make anyone late. The difference is if you miss an intercontinental flight, you lose a lot of money.
This is just friendly advice, not airline policy. Actual check-in cutoff is much closer to departure. Eg, JetBlue is 30 minutes for domestic, 60 minutes for international. Delta's are similar, although it lists a few cities where it strongly suggests to arrive really, really early.
The check-in time assumes that you will check luggage (and not fly with carry-on only). For long-haul super-jumbo aircraft (B747, A380, B773) more time is required to process luggage. For international flights, checked bags undergo more rigorous security screening, which takes more time.
Perhaps more importantly, on international flights, airlines are required to verify that each checked bag is matched to a passenger who is on board. If the airline finds that a passenger checked luggage but didn't board, the airline must retrieve that passenger's bags from the hold. That's why, on international flights, you may hear some passengers being repeatedly paged in the airport, at the gate, and on-board ("If passenger xxx is on board, please ring your flight attendant call button.")