Why?
1) handheld apps are too small to use a dual polarization antenna
2) many apps naturally orient the antenna in the vertical polarization, ie. cell phone with antenna always pointing up
3) In mobile applications where there are lots of reflections and multipath, the polarization shifts to almost anything you want, so it is less important.
Now, in other applications where things are fixed, high quality electronics are used, and the antenna is far above the earth, then polarization is very important. Some telephone backhaul services actually send one set of data on one polarization, and a completely different set of data over the crosspolarization, and the same frequency. There they are really counting on the antenna to cancel the cross-pol interfering signal!