Heathrow is not an easy airport. There are other rather tough, like Tahiti Faaa, for radically other reasons(every route is either too long, or does not have enough demand, or both).
And there are the specials, like London City, who are easy...unless someone already settled there and took all the slots, in which case it's nearly impossible. I'm the one who build a fortress here in last GW3, I had something like 75% of the market there, and as much of available slots. First to succeed here is unmovable. It requires creative scheduling(closes at 2200, grrrrrrr), but works well. But, if someone strong is already there, don't go. There's no place for 2 here.
For the original question, I wonder if the Q400 is not "too small for the route". That's often a killer. In previous GW3, I replaced an A148(70 seats) with a tech stop on the Nantes-Le Lamentin route by a direct 737Max7(125 seats), and LF went from 20% to 90%. Chiang Mai Hong-Kong is 882NM long, but I don't know the "too small limit" for the Q400(never flew those babes). But I know the bigger E190 has the same limit around 1200NM, so I would not be surprised. Just, I cannot check in this world.