My thoughts are that Fallout as a game, whichever one you play, has certain unique qualities which are very appealing and draw you in (the 50's culture, the world, the humour etc.). I played Fallout 3 first and to have all that thrown at me from the off was brilliant, and one of the main reasons I enjoyed it so much.
Most people here kvetching about it put 100 hours into it, so don't feel too defensive. Bethesda's games have a weird draw despite all their shitty aspects, which is something a bunch of us have discussed here before. The fact New Vegas is so respected shows that in the end it was mostly Bethesda's writing, quest design and world building that sucked, as the core gameplay is mostly pretty similar. It's a shame because those are all areas where Morrowind excelled so much, and even despite improvements Bethesda has never gotten close to that again.