This is a confusing question that seems to be asking many things. Try to ask a single clear question.
The answer to the question (which I'm not sure you actually asked) "was the diatonic scale designed to make pianos easier to play" is clearly "no", because the diatonic scale precedes the invention of the piano by some thousands of years.
Remember, for the vast majority of the history of music, it was not played on keyboard instruments. It was played on wind or string instruments. If you want to see instruments on which the chromatic scale is clearly laid out, see the neck of any guitar, ukulele, or other fretted stringed instrument.
The answer to the question (again which was unclear) "why is C sharp enharmonic with D flat" is because it is highly convenient to do so. As other answers have noted, the fundamental relationships in music are ratios of vibrations that are 2 : 11 or 3 : 22. ButBut it is impossible to make any combination of 3:2 ratios that works out to a 2:1 ratio! What we do then is we choose twelve notes that are each in a ratio to each other of the twelfth root of two; that number can be raised to an integer power that gives a result very close to 3:2. I wrote a series of articles about this ten years ago; see http://blogs.msdn.com/b/ericlippert/archive/tags/music/a series of articles about this and startten years ago (start from the bottom).
The answer to your question "could we have a black key between every white key on the piano?" is yes, and this arrangement would have several nice properties including making it trivial to transpose on a piano (by any number of full tones; transposing half tones is tricky in this layout). The traditional piano keyboard arrangement makes it difficult for even experienced pianists to play a piece known in one key in a different key, say, to accommodate the range of a particular singer. The Wikipedia article on isomorphic keyboardsWikipedia article on isomorphic keyboards may be of interest to you. http://en.wikipedia.org/wiki/Isomorphic_keyboard
You may also be interested in studying the key layout of the button accordion.
It would be entertaining to build a small piano or organ which had the keyboard layout you propose, and learn how to play scales and chords on it. If I ever build a keyboard I'll give it a try and report back.
The answer to your question -- which again, I am not 100% sure you actually asked -- "why not just go up whole tones every time and have a six-note scale?" is, you: You go right ahead and play music like that if you want. If you're watching a movie made in the middle part of the 20th century and a character suddenly goes into a dream sequence, odds are pretty decent that the incidental music uses the scale you are describing. Music written in this scale can have an unsettling and dreamlike quality to it, at least to people accustomed to listening to westernWestern music.