Because they're not ready to yet - they will open up when the conditions are right.

They usually open eventually. If the weather is too cold, they may delay. However if the buds turn brown or shrivel up, that's a problem - usually a virus.

welcome to GoY.
It sepends on which ones you are growing. some like the Drawin hybrids are mid to late May. my Queen of the night are not showing any colour yet but they are getting slowly bigger.

Some of mine have flowered & finish & some are still in bud, they will open when they are ready :o))