Here's the actual reasoning for 'Flat' as opposed to radius fingerboards.
On both Classical and Flamenco guitars the most important hand in terms of accuracy (not much in it, granted!) has to be the plucking hand. Classical pieces use a lot of position 1 and position 2 (Fretting Hand) playing which involves a lot of open strings. The plucking hand has to overcome some very technical and intricate plucking patterns. The 'Christ knows' how old concept of a flat fingerboard means the bridge and therefore the strings are flat (Level) at the plucking hand end and assure more accuracy. A radius board on an electric bass would imply a radius setting at the saddles which in turn makes for harder work and compensation for the plucking hand. In todays modern bass playing with players like Matt Garrison, Damien Erskine and Victor Wooten, to name a few, the plucking hand is playing some very intricate patterns so it's easy to see how a flat fingerboard can be a huge advantage as opposed to a radius board, which is probably more to suited to lead guitar making barr chords easier at the fretting hand. As bass players we don't play a lot of barr chords but we do have to pluck some fairly complex patterns. Just have a listen to Damien Erskine's playing to see what I'm talking about.