Maybe it is a couple of things, Buffy was a far more successful show than Dark Angel. Alba wasn't necessarily just known for that role where as SMG was and always will be Buffy. Also the films Alba picked, whether they were cinematically gratifying or not, were hits. Big summer, teenage girl loving hits. Sarah has only one to her name and that's Cruel Intentions. Also I kinda feel that SMG took herself too seriously and picked roles that she really shouldn't have. Then again there are also times when she didnt take herself seriously enough...Scooby Doo anyone?
...plus, she never got to wear a bikini in any of her roles...
No, seriously, I think being identified with a particular role to the exclusion of all else the way Sarah is STILL referred to, or at least associated with, her role as Buffy, has hurt her career the way type casting has done more than a few talented actors. Look at how long it took Sean Connery to get any sort of decent role after he left the James Bond franchise, or John Wayne, who could never make anything believable except westerns and war movies.
After Buffy, Sarah has carefully avoided the "kick ass chick" roles of the "Kill Bill" variety, presumably in order establish herself as a legitimate actor, and as a result, the only things that she HAS done haven't either appealed to or expanded her fan base.
To the best of my knowledge, nobody refers to Jessica as the star of "Dark Angel." Maybe having her show only run for a couple of seasons helped with that, or the fact that in many ways, Max was a copy cat Buffy, and, as such, less remorible than the original.