What is the chemical property that makes it waxy?
Stearic acid. It's very waxy. Candle wax and crayons are made from it.
In cattle, waxy fat contains more stearic acid and less oleic acid. Oleic acid is unsaturated and liquid at room temperature, so the less there is, the harder the fat.
Here's a table that compares fat from three places in the bodies of cattle. As you can see, the waxier fats have about twice as much stearic acid (C18:0) and 25% less oleic acid (C18:1).
Here's a quote from a study about lamb fat:
...perinephric fat [suet] contained more stearic and less oleic, palmitoleic, palmitic and myristic acid than subcutaneous fat.
Here's a table from a different study about lamb. The numbers circled in red show that kidney fat (suet) contains more than twice as much stearic acid as some subcutaneous fat.
Ashes JR, Thompson RH, et al. A comparison of fatty acid profiles and carcass characteristics of feedlot steers fed canola seed and sunflower seed meal supplements protected from metabolism in the rumen. Aust J Agric Res (1993). 44; 1103-12.
L'Estrange JL and Mulvihill TA. A survey of fat characteristics of lamb with particular reference to the soft fat condition in intensively fed lambs. The Journal of Agricultural Science (1975), 84:281-290.
Popova et al. Content and fatty acid composition of different fat depots of lambs receiving fish oil supplemented diet. Bulgarian Journal of Agricultural Science. 2008. 14(1):100-107.