What's better about Fallout 1 and 2 over 3?
The writing, the plot, the characters, dialogue options, freedom of choice, moral dilemma, moral ambiguity, the setting, the diversity, the sense of humor, the wittiness, the attention to detail, the character creation, the rpg mechanics.... I could go on and on, it'd actually be easier to point out what Fallout 3 did better than the other two games.
Let's see. Fallout 3 had more entertaining combat. Not better combat really, but it was certainly flashier. Better graphics, obviously. And.... umm... erm.... that's about it.
Which isn't to say Fallout 3 was bad, not by any means. I actually quite like Bethesda, even if I have been kinda disappointed by their latest works (Morrowind is still my favorite of their games!). But Fallout 3 'as an rpg' was quite poor compared to the original games. There was less freedom, less options, less dialogue and less plot compared to either of the old games. Instead Fallout 3 seemed to focus on elements of gameplay completely different from those of 1 and 2. FO1&2 had a tight focus towards sharp writing and freedom of choice for the player. Fallout 3 meanwhile seems to focus on exploration and combat.
That's about it. If you prefer the elements that Fallout 3 focused on, then you'll probably prefer it over the original games. If however you prefer the elements which the originals focused on, then you, like me, will prefer them. It's that simple.