Conferences, no matter the size all have one thing in common, they all require the same thing to run from Breakfast until Dinner for the day (or multiple days) that the event runs. That thing they need to run on is cash. The less expensive the event is to the attendee, the more the team running the event will need to make up this cash from somewhere else, usually from Sponsors.
Can an event run without sponsors? Sure. But say goodbye to coffee, snacks, sodas in the afternoon, possibly lunch. And in most places, say goodbye to the venue. These are all the things that sponsors are paying for by showing up and being there; among potentially others.
What it boils down to, is at events, especially smaller events, thank the vendors. They gave up their time and their companies cash to talk to you.