Because the convert holders don't put their shares up for sale. They let the MM's know they'll cover any shorts because they can buy from the company at a discount to whatever the price is.
So here's how it works. Newbie Investor comes along, says, "Hey, I'll buy 100k shares at 2 cents". MM sells them to him. Reg Sho records short. MM contacts convert holder and buys 100k shares at 1.9 cents/share. Convert holder buys 100k shares from company at 1.1 cents. Everyone wins, except Newbie Investor who can't complete with convert holders who get a 55% discount.