Organisers are clueless, but that doesn't necessarily make them entirely wrong
The way the organisers have handled this is objectively clearly wrong. The communication and way the talks were organised could have been better, and this is their third year so they should have some idea how to approach this. The fact their website now has a link to three twitter posts and a cancelled message is the height of pique.
However, when one of the speakers that pulls out notes that when they ran a conference even after outreaching to various groups and actively soliciting contributions to improve diversity rather than just blindly asking for contributions, and STILL getting no or practically zero contributions, you have to consider that perhaps their expectations are a little unreasonable.
It certainly isn't easy to achieve diversity targets. Outreach is necessary, things such as travel assistance, income based tiered pricing, on-site creches, and accessible venues all help improve the mixture of attendees.
However, this all costs money and effort, and if it's successfully achieved expectations only increase. Sometimes expectations surpass available resource, so the choice is either that the event does not run at all (as in this case), or that the event runs in a lesser fashion than desired. It's up to the community where their priorities lie.
At least in this event, the withdrawing presenters do seem to be trying to help, and the conference organisers were churlish not to work with them properly. I've been to other events where the amount of criticism became so high that the organisers could no longer be bothered to run the event and surprise, surprise, the people complaining had no sensible plan to achieve their aims.
The one exception that stands out is the BBC decree that all comedy shows would have a woman on the panel. Decent comediennes clearly exist, and it pushed them to improve not only diversity, but the pool of comedians shows drew from.