Attraction does not always disappear after commitment. It often changes shape.
Early attraction is helped by uncertainty, anticipation and limited access. You do not know everything about the person. Time together feels chosen because it is not yet built into the structure of daily life.
Commitment changes the environment.
You see each other tired.
You discuss bills.
You watch each other clean the kitchen.
You become part of each other's routines, responsibilities and stress.
The question is not, "Why do I not feel exactly as I did at the beginning?"
The better question is, "What conditions help desire exist between us now?"