Why don't most academic conferences make their budget publicly available?
I don't think there's any universal policy behind this.  One reason is lack of interest: in many cases I'd bet nobody involved has thought much about the issue, the world isn't clamoring for the budget, and publicly releasing it would take at least a little work.  Inertia is a common reason for not doing things.

Another reason is to avoid arguments.  The main thing people would do with a publicly available budget is criticize it.  Some criticisms would be based on ignorance of the options and constraints, while others would amount to saying "more should be spent on my favorite things, and less on the things I don't care about" (which is not so useful if there's no consensus as to which aspects are more valuable).  E.g., is money for snacks essential or wasteful?  What if they are overpriced hotel snacks, but there's no other option without moving to a venue that is worse overall?  What about a reception with alcohol?  Basically, imagine repeating every discussion the organizers had about the budget, but with an unmanageably large group arguing, and with random trolls from the internet chiming in.

In general, I would assume it's not an attempt to preserve secrets or disguise unpopular spending.  If the conference is funded by a grant, then the funding agency saw a budget.  If it's funded by registration fees, then participants at least know how large those fees are and can compare them with other conferences.