Conferences are social gathering events. They are better attended if there are organized in better and more accessible places. Taking care of everything at conference, and do it at some better location has a cost.

Some conferences are institutions, have a formal organization that stands behind them and it is well organized so has some of these costs figured out and planned well in advance. Many are not. Many conferences are organized by different institutions each year, and are organized in turns by the hosting institution at locally available venues.

Imagine a small conference organized by a small department and it's total budget:
- 100 presenters * $500 registration + 100 attendees * $100 registration = $60000

You need a venue. Many departments do not have resources to organize conferences in a presentable manner (lack of space, lack of rooms/halls). So you need a venue. Plus if the conference is in a nice venue, for a cheaper price - it will certainly be attended more.

Let's say that you were lucky and that some hotel accepted to be exclusive venue and they offered all of their equipment and halls, if at least 50 participants decided to book a room for the duration of the entire conference and there were 50 such participants. That way, you don't have to rent the hotel's halls and their projectors/laptops/screens/microphones.

You will still need to pay for this:

- costs to get respectable keynote lecturers and pay for their travel fees, room, meals, etc.

- organizing teams costs (even if they volunteer their time, they still have traveling expenses to visit the venue at least several times and organize local issues)

- hosting for a dedicated web-site for many years to come + web designer to support it 24/7 during the call for papers and during the conference

- print/publish the proceedings + editor

- venue for either the opening or the closing ceremony or some joint excursion or museum visit or all of these if you want to leave an impressiong that you are a host that wants to pamper their guests (if you don't pay for at least some of these, most people will not come next year, as the point of the event is to be a socializing trigger, and socialization always happens in the margins of the conference, at these social events, so the whole point of the conference will be lost if you don't take care for at least several such events)

- at least 2/3 photographers to cover social events and parallel sessions

- finger food / coffee / drinks / even water during breaks, maybe lunch for all participants

- banners, badges, posters, ...

I have organized an international conference and I can say that this is just the surface. If you organize an IT conference, you need to take into account that each participant will have at least 2 online devices, maybe more. So you will need wifi access points and routers capable of peaks in new wifi associations ranging from 200-300 per minute, or more. You will need a lot of electricity cables and plugs, power convereters etc. Most hotels doing small gatherings for 100-200 persons, are in fact organized to accomodate weddings and not IT professionals and their net streaming needs.

Do you own total, depending on wherever you are. In some countries this will cost much more than the budget, in some less so their might be something left to pay the organizers and even volunteers for their effort. Not much, but at least something so they do not avoid you when you decide to organize another conference in two years.