I don't accept the "prosperity gospel" If it were true, these preachers could keep writing their own checks & get 10 fold back without ever asking anyone else for their money.
The Kingdom halls are full of low people who have to resort to whatever jobs they can get that don't intervere with meetings, field service, assemblies & conventions. They can't get a college degree, they can't work overtime, they can't do anything holiday related, they can't carry a weapon. This kind of narrows the job pool, so I would say that is why they struggle with finances, while some other groups seem to prosper.