From what you hear it’s that they have bills to pay. If a publisher doesn’t know your name, they’re not going to assume it’s an automatic best-seller so they’re less likely to actually look at your work to see if it’s worth it.

Also, to pay your bills before the publisher actually prints the book, you have to find one way ahead of you finishing the book, meaning that you have to send them an unfinished manuscript, further hurting your chances - but if they like it you get that money before it’s done, with the caveat that they’re gonna sue if you don’t deliver.