A while back I organized a large developer day conference session (for a non-profit). We went through wrenching early debates over speaker subsidies. Which we needed to clarify up front, as any lost revenue from subsidies needed to be recovered by raising overall conference fees.

Indeed if you want to be as inexpensive as possible for the largest number of participants, you don’t subsidize.

Also, it turns out conference budgeting & forecasting is complicated, and it can be hard to break even at the best of times. So organizations that can’t afford deficits - like non-profits or small charities, will lean towards the (simplest) model that is most easily predictable and minimizes financial risk. Namely - no subsidies.

In my case, as a non-profit, we could not risk a deficit and wanted to minimize the cost of entry, went with no subsidy. This also made the conference, for participants, as accessible as possible, which was also a goal. I believe the organizer of this event (the Anita Borg institute, along with the ACM) is also a charitable non-profit, and likely followed a similar logic.