Because at slow speeds you are not light/weightless/floating during the transition and will end up in the backseat if you flex. Which is why you should not learn a beginner how to flex imho.
You need a certain level of performance in your turn in order to perform flexing corrrectly. That level of performance is not there if you are going slow.
This depends on what you mean by "slow." If you mean almost not moving, as on the beginner learning hill, yes, it's difficult to gain enough momentum to result in the body toppling across the skis when one flexes the new inside leg to release. Teaching an extension release to beginners going super slow works much better.
But if you mean slow-ish on blues or greens, I'm surprised that you would say flexing to release does not work at such speeds at all. It does. Momentum carries the body across the skis when the outside leg is flexed, and the new turn happens quite nicely. Yes, the CoM will be momentarily behind the feet as the skis shoot outward in the top half of the turn, but those skis do catch up with the body without any issue, and in my experience the skis are not
heavily weighted through the top of the turn anyway. This is not a situation where the skier is in the back seat because of lack of skill, nor does it take any particular correction movement to get the skier non-aft. It just happens as the skis come around. Such flexion turns are a good thing to teach intermediate skiers who wish to move downhill at moderately slow speeds. IMO they are as ready as they ever will be.
Is a flexion release hard to teach to an intermediate who is used to extending and pivoting the flattened skis? Yes, it is. But that's because the embedded habit persists so strongly, not because the new movement pattern won't work at slow-ish speeds.