I'd assume because 20MP has hardly acceptable quality due to ridiculous amount of noise. mind that 20 MP is the pixel count of professional full frame cameras like Canon 5d mk2. on FF the count adds some quality, on microscopic phone sensors it just adds noise (not to mention that to have a lens that can benefit from 20MP you would have to spend several times more than the cost of xperia z1). limiting photo size to 8mp allows some better noise handling, as data from several pixels can be averaged.
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