I'm sure nostalgia goggles is part of it (and I am well aware that many voices from the original series were also just phoning it in), but it seems like these days they don't even TRY to make them sound like an animal's cry.
I wish they (US and JP) would modify the voices like they do with Charizard more often. It is actually saying "Lizardon" but it's so distorted you can barely tell it's not just a roar.
I always wanted Deoxys to sound like Shuma-Gorath.
http://youtu.be/ufjRI7yhEf0?t=2m36s