Probably compactness for packaging... Most pistol caliber carbines, similar to AR15s, the 20 round mags extend to a length roughly equivalent to the length of the pistol grip, whereas the 30 round mags extend further.
IIRC, Maryland and Virginia are the only states with any mag restrictions at the 20 round threshold...
CZ-UB lists 30 round mags as standard: http://www.czub.cz/en/produkty/samopaly/cz-scorpion-evo-3-a1.html
As to Virginia & magazine capacity... The Code of Virginia states that it is unlawful to carry a loaded semi-automatic center-fire rifle or pistol equipped with a magazine that will hold more than 20 rounds or a shotgun with a magazine that will hold more than 7 shells ?in the Cities of Alexandria, Chesapeake, Fairfax, Falls Church, Newport News, Norfolk, Richmond, or Virginia Beach or in the Counties of Arlington, Fairfax, Henrico, Loudoun, or Prince William.
Please note, it says to CARRY, not POSSESS. So if you want to CARRY a loaded rifle/pistol in these places listed (not all of VA.) it must have a 20 round or less magazine OR you must have a CHP, which allows you to carry any size magazine you choose.
As to why CZ ships with 20's - not sure, but I can carry a loaded 20 rounder in my Scorpion and still have it fit in the small backpack I use as a carry bag for it. Another advantage is it's easier to shoot from a bench rest position with the shorter 20 round mag.
Have a good weekend,
Bob S.