Less hassle. Shareholders can be a pain in the backside (deservedly so, after all they normally pay for a companies expansion in the first place) There's more flexibility within a private company where major decisions are made by a small number of people and aren't subject to delays and expense obtaining shareholder permission. Also no arseing around complying with the many listing rules, which also costs money.
I stress this is all my opinion. It's an (I'd like to think) educated guess and not born of personal experience.