Private servers are running on emulators, they are far more behind than what we playing on retail version at the moment, WoW being the biggest MMO has many things that Private server Emulators cant handle, every class has different dynamics and there are nearly 12 classes with 3 spec each that makes 36 spec in total, and balancing those 36 spec's is nearly impossible for private emulators.
thats just one thing I have mentioned there are several other aspects, raids/pvp/exploration/quests/professions/NPC's/dynamic enviroment etc etc etc you just cant find them working as retail on Private servers.
population is one of the main reasons on Retail WoW there are around 10-15k people on each realm and there are 100+ realms on EU only aint talking about US and China, on Pserver u will find not more than 3k+ people and that 3k will drain Pserver like a dork suckin machine resulting in lag and high latency.
well i can write a book on the difference b/w both, but I hope that suffices you need of clarification.