Hmm...fallout 1 was good. It was also more balanced than 2.
However, what lacked in fallout1 was the humor that was in fallout2. Fallout2 seemed to have much more humor. It made me laugh quite a few times as i went through the game. I guess that's mostly subjective.
Perhaps why most remember it as "the rpg" was because it contained many elements...humor, sex, action, twisted stories. Pretty much everything, and suprisingly did pretty well in every aspect.
The first one was more balanced combat wise, had a good story, and was overall - a really good rpg. Just like many good rpgs out there, they all had a good combat system, a great storyline, and was overall, really great.
To most, the second was more memorable due to the diversity of content. That's perhaps why it's referred to constantly as "the rpg"? I'm making guesses here. If you have a better explanation as to why it's constantly referred to as what it is, i'll be happy to listen. =)