Yes, as the airlines have to check-in another 100-300 passenger as well... Not only you, so by setting the limit to 2hours... For those who arrive early, then they could check-in first and relax... And for those who arrive at the 2hours limit, then there would usually be a queue there waiting to check-in as thats when most of the people arrive... And it may then take up to 30-45minutes to check everyone in... By which time, it would already 1hour before the flight depart...
To ensure that all the check-in luggages get the necessary security check and to the aircraft on time, it is necessary to get passenger to check-in early so even if there is any delay, it would delay the flight by a long time...
And of course, you can say that since you will have to wait for like 30-45minutes if you are not flying with those airlines who operate loads of flights to Hong Kong... Why dont you just turn up later??? Well... By setting the limit to 2hours, there is another purpose for the airlines as then they can count the number of no-show passengers and the airlines will give those no-show passenger's ticket to those who are on waitlist if the flight is full... Otherwise the information of luggage weight and passenger number will then be sent to the pilot as the flight plan so that the captain would have around 1hour to caculate the necessary information and see if they would require more fuel or not and all those other necessary things that need to be done before the flight depart...
So it is very important that you arrive at the airport around when the check-in desk open to be sure that you seat wont be sold to waitlist or standby passengers and I would say for the latest, no later than 1hour 30minutes before the flight depart so that if anything goes wrong, there is still time... As airlines usually close their check-in desk 45minutes to 1hour before flight depart...