Why are backhand clears difficult?

You only play them when you’re already in trouble!

Good players will avoid backhand clears when possible, and play round-the-head forehands instead. This means backhand clears are only played when you’re under pressure.
Specifically, we only play backhand clears when the shuttle is out to the side, and usually also slightly behind us. This is a difficult, defensive position.
How good are your forehand clears from that position?

You don’t practise them as much

Since players avoid backhand clears, these shots get much less practice during games than forehands do. A good doubles player will hit at least 10 times as many rearcourt forehands as backhands.
You probably have much more experience hitting forehand clears than backhand clears. That experience makes forehands easier.

They are physically more difficult

The shoulder is much less mobile when making a backhand hitting action, compared to a forehand action.
Loosely speaking, the shoulder can go farther forwards than backwards. When you’re playing a forehand, your shoulder movement can continue after impact with the shuttle: the impact point is somewhere in the middle of the shoulder’s range of motion.
With a backhand, the impact point is near the end of the shoulder’s range of motion. This limits the amount of force that the shoulder can contribute to the shot.
A backhand clear action is also less natural than a forehand. It’s a movement you would almost never make except in racket sports.

You need realistic expectations

Sometimes it’s just not possible to play a good clear, because you are under far too much pressure. Players tend to blame the last shot of the rally, when often they should be thinking about how they got into that situation.
It is realistic to clear end-to-end on your backhand, even with decent height, and even cross-court. It’s not easy, but it’s something you can learn.
It is not realistic to play a good clear when you’re desperately late to the shuttle, reaching behind you at full stretch, and off-balance.