I haven't had Wifi in any of my flights between Thailand and Amsterdam (KLM). Maybe I've always had older planes on this route or something. I haven't flown between any other 2 destinations for the last couple of years.
It really depends on the airline, but it's fairly common to put newer aircraft with WiFi on more business-heavy routes, and put the older aircraft on more tourist-heavy routes where passengers are more cost conscious and probably won't pay for the WiFi or other amenities either.