Excellent question. Took me a while to figure it out.
Ionotropes are contraindicated for obvious reasons. The heart is already working hard and you don't want to make it worse.
The reason why ACE inhibitors and nitrates are contraindicated like in the concepts of manoeuvres for murmurs.
The murmur is intensified when there is decreased preload causing increased obstruction. And ACE inhibitors and nitrates reduce preload, hence increased obstruction and reduced cardiac output.