Some partners don't reveal it to their spouse because they feel they could be judged wrongly due to the kind of person they know their spouse to be as someone who doesn't like gambling. But even in relationships we have to respect each others certain choices in as much as they doing those things in a responsible manner.
I may not support support the kind of behavior whereby those in a relationship are not being opened to each other, at least to an extent, we should be able to say one or two things that our partner is into, not because we want to mock them about it, but because we also wanted to ensure that in everything, there's utmost transparency in knowing about what each other is capable of and upto, because we cannot hide things from each other for life, later everything will got exposed and that would be more worse scenario when our partner got to discover something like gambling.
Actually in every relationship transparency and sharing of secrets are keys to a strong bound leading to better understanding between the two, but when you have a spouse that's somehow judgemental at almost everything, you will surely be unwilling to reveal certain personal things about yourself to them especially gambling that is being vilified by many as something bad.