High pitch "voices" (or, better, voices with higher harmonics that are more predominant) are normally more "interesting" to the average listener.
It's more or less the same thing as "fast is more than slow". A fast driver usually gets more attention than a cautious driver, no matter which one is "best".
We're normally more "captured" by physical extremities, as they are the easiest way to recognize something.
Within the same family of instruments (or even the same instruments, like saxophones), we're usually more captured by higher pitches/harmonics.
We can see a similar pattern in human voices: there are more arias for sopranos and tenores than for altos or basses (though, a bass/baritone voice is usually more peculiar - hence, interesting in its way). And it's no mistery that castratos were a common occurrence in the past, due to the high-pitched and "angelic" (as in more "consistent" harmonics) sound of their voices.
Now, one could argue that, given the above, cellos shouldn't have that attention. But you should also consider other aspects: while cello is very similar to a Violin (or a Viola) in construction and physical emission, its technique, size and range are very different. They are the "tenor" to singing as "sopranos" are. Within their "group", they are more "interesting" (and much closer to human voice, which is an important aspect). Also, the very technical and physical nature of a doublebass create serious obstacles in playing virtuoso parts or making higher pitched sounds that are gladly perceived by the normal listener, as opposed to cellos.
Obviously, there are other aspects to consider too (as others already pointed out), including the recursivity of being considered a "minor" instrument: a composer usually tends to write for "major" (popular) instruments, which automatically increases the "minority" consideration for the others. They also usually know less about those instruments for the same reason, so they hardly choose to write for them.
Finally, Viola (as much as Alto in voices) is probably the less considered voice: it's not "on top" among its "siblings", and it's not even any of the extremities. This obviously results (and has resulted) in minor interest of players in improving their virtuoso abilities, as ~80% of Viola parts are technically not that challenging (or "interesting") as Violin parts usually are.
Which recalls the above recursion: players are not "that good", so why should a composer put any efforts on that? (rethorical question).
But that's the good of it: there are very few Viola specific compositions, concertos or solos, as opposed to Violin, which makes them even more interesting. They are rare gems.
For instance, after years of listening to Cello/Violin concerts and sonatas, I discovered the first Viola Sonata from Hindemith, and I instantly fell in love with it.
 Soprano Sax is usually considered much harder to play than Alto, as the smaller reed creates more tuning difficulties. While the same could be told about vocal chords, the difference is being "born with an instrument" (as in voice) and learning to master it.
 Obviously, I'm not considering 2nd Violin parts, since they're still Violins. And, obviously again, I'm not ignoring them just because they're second. ;-)