From E to Eb gives the instrument a range of 36 notes, the exact number necessary to be able to play each of the 12 notes in 3 different octaves. You have your high/medium/low E, your high/medium/low F, your high/medium/low F#/Gb, and so forth. So it is equally easy to play in any key, with 3 octaves to choose from.
Adding more frets creates a weird situation where the instrument has more range in certain keys than in others. That said, I realize that a lot of us are rock or blues players and frequently play in the key of E. So maybe there is a legitimate argument for an instrument that has more E's than any other note? Certainly horn/brass/woodwind players are no strangers to the concept of an instrument that favors certain keys above others.
However, I would counter-argue that making the electric bass key-agnostic (equally able to play any of the 12 keys) was one of Leo Fender's key contributions to 20th century music. Unlike a big band or orchestra, an ensemble of piano/keyboard + electric guitar + Fender bass can easily play together in tune, in any key, allowing producers to arrange sessions in the best key for the singer's voice, which I think changed the history of pop music.