If he still had his memory, he would know what planet he's on in Star Wars, wouldn't he? Not to mention recognizing people and places there.

Threepio knows too much, and he loves to talk. Most people in the SW universe do not get very attached to their droids to begin with. And a Lobotomy isn't the same thing as losing your memory.

Perhaps protocol droid personality is hard wired? (Do amnesiacs develop a whole new personality?) Doesn't explain why Artoo gets a pass on the memory wipe though.