Well, for the Sprint version, first reason is their opting to stick with the Korean build without SD card slot.
A lot of people don't care for non-removable battery setups.
This phone had lots of potential, especially with the early leaks, but from what I gathered, LG has developed a similar reputation to what Samsung used to have... Lack of OEM support.
Another reason it's not getting the following it should is timing. With the Note 2 finally set to be released on all carriers in the US, coupled with Verizon and the Droid Incredible X, the Optimus G is getting lost in the background.
Up until they announced that Sprint's version would be internal storage only, this was a strong contender for me. LG should have jumped on the opportunity to release the phone worldwide after the Korean release.
They had a display (sprint) version at best buy that I checked out. It is nice, but the Note 2 features were too much for me to ignore.