When businesses need to raise money for new projects one popular route is to hold a public share offering.
As this would by definition decrease the % and hence value of the shares held by the current share holders, usually current shareholders are offered the chance to buy into the new stock, with the rest being sold to new buyers.
The only reasons I can think of that this hasn't been employed is that not enough of the current shareholders are willing to risk either putting more money into the club or see the value of their current stake decreased in the short term against a possible rise in value over the medium term as the money raised is used to invest in 'projects' which increase the club's overall worth.
Or put more bluntly, the current shareholders are not optimistic about the club's future.