They are simply protective of their customers. They will list good projects when they think it will provide volumne on their platform - aka their customers would want to truly buy them.
But just because you throw money around, you cannot simply buy access, otherwise the spammier the project, the easier it gets.
I know from lower tiered exchanges that is it easier to 'get listed' aka buy in.
Given also how much binance is a boost ('we got listed on binance') and the amount of new tokens every day, this is likely more of an administrative hurdle.
It's the same principle as in the book industry. The best available promotion machine with proven results is bookbub. To get a bookbub slot often guarantees you NYT bestseller status and more. Tons of paid and free downloads. And they charge a pretty penny. However - they will only list books which have the potential to do well with their audience. You cannot buy access, only pay for the privilige.