Because men don't wear nightgowns ! He's a man with a wife ! If he was gay, it might be different, but I can't think of any reason why a man would wear a nightgown!
Why shouldn't I be able to make my husband wear nightgowns for me?
'Cause they're men.
You shouldn't be able to make your husband do anything that he doesn't want to do. Although it would be absolutely fine for a male to wear a nightgown if he wanted to do so, it's unfair of you to expect him to simply because it's what you want.
Being in a relationship doesn't mean that you've got full control of what your partner does or doesn't do. There needs to be a balance of compromise and self-ownership.
How would you feel if your husband wanted to force you to wear something that you were unconformable in? I'm sure you'd agree that this was a little unfair of him.
Likewise, when you say that you should be able to dictate what your husband wears, you're attempting to remove his freedom to decide for himself. Ultimately, in most instances this is a path that will lead to an unhappy relationship.
Sexism
I do, however, feel that a man should be able to wear what he wants. To suggest that he shouldn't wear something simply because he's a man is pretty sexist. It's 2015, and I think we're rightly moving away from previous "gender roles".
That being said, it all comes down to personal preference. Your husband has every right to refuse to adhere to your demands.