Why are some plants frost tender?
All cells are susceptible to cold, but freezing is particularly difficult.  If the water in the cells freeze, it would usually burst the cells (water expands when it turns to ice) and tearing the cell membrane (totally killing it).  

Some plants and animals have adapted to colder temperatures by putting antifreeze proteins in their cells.  A lot like adding antifreeze to the radiator of a car, this lowers the freezing temperature of the water in the cell.  

So the answer is that some plants have adapted to colder climates with the help of additional genes.  There are other things that have to change too - the chemical reactions in the cell will go haywire when temperature changes too, so lots of genes must make adjustment for a fully cold adapted plant.