Standard doorways exterior doors are 36 inches wide (although local codes may not require all exterior doors to be that wide), while most interior doors are 26-30 inches wide. Average arm lengths are much less than 36". It's therefore much less convenient and much more difficult to deal with a tanks of that width. As a result most people only see tanks in the common narrower widths and that becomes the standard size. The same factors affect the height (and in some cases compound the effect.)