A fantastic thread has developed on another forum I use (Elite: Dangerous the space trading/shooting/pirating/stuff game) where somebody commented on a spelling error in the recent release (its an English software house), which gave rise to 6 pages (and counting) of explanations about the US and the metric/imperial measurement system and the 'odd' spelling of some words.
This was offered up by one of our 'Murican (evolved to 'Merkin' in the thread as it goes...) cousins by way of explanation of the missing vowels and other letters...
In America, our U's are very carefully rationed. We have to allocate a set number to continually chant "USA! USA!" and so on. We can't just be throwing them all willy-nilly into other words. S's must follow similar rules and that is why we use -ize instead of -ise. "USA" can reasonably be shortened to "US" so A's are more freely used.