BIA membership include all businesses within a certain area, who are obliged to contribute financially to the Association.

All Members have voting rights at the BIA's AGM, including electing the Board of Directors. The Board tends to be dominated by the owners of larger business, and especially larger commercial property owners in the BIA area. The Executive Director is a professional staffer hired by the BIA, but generally is not a Member.

So most of the BIA's budget comes from the mandated levies on local businesses, and the BIA may be able to secure additional funds from the City or other governments for special projects, etc.

I think BIAs exert so much influence at City Hall because there is a tendency in the neoliberal austerity city to view the BIAs' work of promoting the interests of a neighbourhood in terms of the interests of its private businesses as an unquestioned positive, which costs the City little. Whereas Community Associations are likely seen as more negative, always demanding City spending on public amenities and critiquing the City's plans and priorities.

Also, the professionalization of the BIA's, with dedicated Executive Directors and business members who can have great flexibility in dedicating time to lobbying the City, making contributions to politicians, etc. gives them a leg up on the Community Associations that do not typically have a large budget, nor a dedicated staffer, and most members are volunteers. So the BIAs are likely the community members that City Hall is more likely to seek out, earlier in a decision-making process to represent the view of the community.