Because Zekrom is more viable than Reshiram.
I'm comparing them for BW, Ubers. About Reshiram:
Although its dual STABs are notorious for being unresisted, Reshiram's reliance on weather, lack of defensive utility, and susceptibility to hazards make it a lesser player on the Ubers scene. Additionally, Reshiram has to compete with Kyurem-W for the position of "Dragon-type special nuke." In the minds of many, Kyurem-W's extra power, better Speed tier, and lack of weather dependence overshadow the ridiculous power of Reshiram's sun-boosted Blue Flare. The introduction of Regenerator Ho-Oh causes more problems. If Stealth Rock isn't up, Ho-Oh can now switch into a Choice Specs Draco Meteor and switch out to a resist on the second hit; any other move on the Choice Specs set—save for the uncommon Stone Edge—can be easily weathered by the flaming turkey. Taking into account all of these drawbacks, one might wonder if there is any reason to consider using it in BW2.
As you can see, Reshiram is outclassed by Kyurem-W (and my favorite Pokemon :D). Kyogre is very common in Ubers, so it is important for a physical electric attacker. Apart from that, Kyurem-B wasn't that good as Zekrom. There aren't many good electric-types, so Zekrom is helpful than Reshiram. He also doesn't get guaranteed OHKOed by Kyogre's Ice Beam, here you can see:
252+ SpA Kyogre Ice Beam vs. 0 HP / 0 SpD Zekrom: 240-284 (70.3 - 83.2%) -- guaranteed 2HKO
While:
252 Atk Teravolt Zekrom Bolt Strike vs. 0 HP / 4 Def Kyogre: 512-606 (150.1 - 177.7%) -- guaranteed OHKO
Thus, it is important to run a solid Kyogre check or counter.
About Zekrom.:
> With a monstrously high base 150 Attack stat, combined with powerful and complementary Electric- and Dragon-type STAB moves, Zekrom has the power to devastate many teams. Zekrom's signature move, Bolt Strike, will leave enormous dents in even those who resist it, and forces people to run a Ground-type Pokemon or a very bulky Electric-type resistor in their teams. Even then, they will have to tread carefully against its extremely powerful Dragon-type STAB moves. While Zekrom does not have much to offer defensively, it is an excellent check to Ho-Oh and defensive Kyogre due to its resistances to Fire-, Flying-, and Water-type moves.
