I would suggest that the very purpose of a stock market listing is to create a market to allow the easy buying and selling of shares in a company. The Mantegazza family might wish to sell their shareholding in Monarch but they will find it extremely difficult to find a single buyer who will offer a reasonable price. By contrast a pension fund with a 0.5 % shareholding of Easyjet can readily find a group of disparate buyers who combined will take up the pension fund's current stake.
A company with just a single shareholding can keep all meetings behind owner and directors behind closed doors. A plc listed on a stock market by necessity has to make its biannual meeting of directors and shareholders a very public affair. Anticipation of squirming in the public spotlight tends to make directors think very carefully about how best to run the company, and, more importantly, how to communicate that strategy to shareholders.
The sole recourse the Mantegazza family has for the time being in the event of a profit warning, is to fire the current directors and hire new ones - selling a large chunk of their shareholding just isn't possible. If shareholders in a plc listed on a stock exchange lose confidence, they will not just acknowledge a profit warning - they will rapidly reach for the "Sell" button on their broker's website.