I think this was asked before (in FT, not sure the subforum) and there was a user working in BKK airport that explained the issue. According to him the problem with early check is the baggage staging area.
In EK's case, and according to him, there's a permanent baggage staging area where luggage can be dropped and stored even if your flight is not in the next 3 hours.
As for other companies, like QR I think, the baggage area is only assigned for a period of time and when that ends another company will take that same baggage area, making it impossible for the next company after QR to accept luggage before official check in time.
In my case, I did in fact drop my luggage in the check-in counters of BKK around ~5h before departure and the agent said it was ok. But my flight was at 2:40 AM and there were like 2 or 3 other EK flights to Dubai before mine so the check in was open for that route.