1. Size matters
The bigger the conference, the more complex the sale cycle. There are a billion moving pieces, and people get progressively more stressed out closer to the conference, because everything has to be executed and be perfect on a small number of days. If you show up during the planning stage too early, they don't know what technology to use yet, so you have to wait till the right time to show up, when they have the topics, speakers more confirmed. Be very aware of where they are in the planning cycle, because budget opens up or not depending on the time.
2. Degree of tech savviness/budget
Every conference has a different degree of savviness towards what tech platform they are comfortable with. There is not really a test run they can do at this scale - so you need to make them feel very confident that the tech will work live for everyone at the same time since there is no gradual scaling up.
3. Pre-/post-conference engagement is a common pain point
The pitch that really got our product sold was to emphasize the continuous nature of engaging with the conference audience beyond those 2-3 days of the conference. Not sure what your product does (mine is an idea sharing platform: agora.co), but that pain point resonates with everyone I've talked to who organizes conferences
Hope this helps!