I suspect that its a combination of the volatility of the market, and the type of markets they are serving and the fact that the loss of perhaps just one major contract could stifle their cash flow to the extent that they stop trading.
PGT have been quite successful over the last few years, but so have several other airlines who served exactly the same type of customers as them and then lost it all overnight.
As DJ6 intimated, if your risk makes the insurers risk untenable, you won't get cover from anyone.