Why is it by many that a Jews religious status is defined by him Keeping shobbos and not by any other misvah and when they refer to such Jews very commonly it is said that he is a shomer shobbos? What makes shobbos different than all other misvot in the Torah ?

Shabbos is the one most important mitsvot that if one doesn’t keep he is considered to be like a goi (also avoda zara).
Rashi in chulin 5 says that a person who violated idol worship is כופר in Hashem and one who doesn’t keep shabbos is כופר in Him creating the world since shabbos represents the creation of the world.