People need to use Rotom-C more to stop Volt-Turn. It resists Rotom-W moves bar the rare HP Ice/Fire. Scizor takes a huge chunk of health switching in to Specs Volt-Switch. I run one with Trick / Volt Switch / HP Ice / Leaf Storm with a good success.
The problem with Rotom-C is that Landorus is very common in Volt-Turn chains, and his U-Turn wrecks you. Yeah Latias and Celebi havie this problem too, but at least they have reliable recovery...
Also Zapdos doesn't really benefit from sun. He doesn't get any notable OHKO or 2HKO with Heat Wave so no real advantage.
In the other hand Zapdos in rain is a beast, even more with Lightningrod. In rain he can use Thunder, which simply destroyes, and he is a ridiculously good Ttar bait which is easily 2HKOed by HP Fighting and Expert Belt. The other 2 moves should be Roost for healing and Volt-Switch for scouting. He can also quite effectively bluff a Choice item with Expert Belt, as most of the time you will be spamming Volt Switch and Thunder.
Why use him over Rotom-W? Ok for the DW Zapdos there is no need for an answer. But for the one we have now, here is the reasoning:
1. Zapdos can actually lure Ttar, while Rotom-W can't, allowing you to win weather wars much easily.
2. Zapdos has reliable recovery
3. Zapdos Thunder simply destroys, as 20 more base SpA really makes a difference. For example even without HP Ice, Haxorus is outsped and 2HKOed by Modest Expert Belt Thunder after SR. Same goes for Hydreigon, which is 2HKOed by a combination of Thunder and HP Fighting after SR.
4. Zapdos provides a useful Fighting and Bug resistance, and is almost a perfect Scizor counter, unlike Rotom-W. I didn't mention Zapdos's Grass resistance since he can't really beat any OU grass type IN rain, except Ferrothorn. And anyway i don't like the idea of running Heat Wave on Zapdos in a rain team just for 1 poke .
5. Zapdos is faster. This means that some threats like Lucario, Haxorus and Hydreigon can't revenge kill you without a Scarf. Also Modest Zapdos outspeeds Timid Rotom-W by 1 point! (not that it matters, since Scarf-less Timid Rotom-W is pretty rare, just for the laughs)
Of 'course Zapdos has its drawbacks in comparison to Rotom-W as we all know, such as SR weakness, lack of Water typing and Water moves and lack of WoW.