As I understand it, most of the ad slots were sold at a yearly sales event. This time most advertisers were cautious because the company couldn't respond to questions of how Elon Musk's takeover would affect the company (and specifically the type of content alongside which the ads would appear) as this was happening before he took over but after the sale was practically a done deal. This left them with only a fraction of the ad slots "pre-sold" compared to a normal year.

Additionally it seems a lot of advertisers have cautioned their ad departments to halt spending on Twitter ads for now because they're still unclear about wtf Elon Musk is up to (not to mention the mass impersonation fiasco that allegedly resulted in share prices of several companies being affected negatively).

It's not just sales people, Twitter seems to have gotten rid of most of its external communications people to the point where journalists can't even find anyone to ask for statements other than Elon Musk himself (which may be intentional given that he likes to present himself as the face of his companies).

But if Twitter's ad sales really did rely so much on these sales events, I'd wager that the sales people largely existed to communicate directly with long-standing big-spend advertising partners. It's hard to overstate how much these types of deals hinge on "social glue" rather than cold hard numbers (which can be interpreted/reframed to be interpreted however you want as long as both sides are happy).