Preferred stock owners get fixed dividends, but no ownership, so their only value is the dividends that they receive. Their value does not go up if the company grows, unlike common stock, which has an ownership claim on assets and a possibility of higher dividends. They instead fluctuate in value with interest rates, since with higher interest rates investors require a higher yield and will not pay as much for fixed payments.
Also, unlike bonds, they don't get their money back unless they sell the stock to someone else, so if the company goes bankrupt they don't get their investment back (unless there are enough assets to satisfy all of the bondholders and creditors first).