The Question should more be: why is iOS 7 not updated.
There are many users still on the (actively being sold) iPhone 3S and 4. They can't upgrade and are vulnerable now. Check?
Where are those phones being actively sold by Apple?
In The Netherlands, at Albert Heijn (big supermarket and mobile network) as 'refurbished new'. Probably not by Apple themselves, but their not stopping it either.
At Telfort, another dutch mobile network,
https://shop.telfort.nl/telefoons/Apple/Apple-iPhone-4-8GB-black-Apple-Certified-Preown.html? (certified by Apple, according to this big operator; and have fun: it's promoted here on the ad-space on macrumours if you once visited that website)
And at a lot of other places
http://tweakers.net/pricewatch/293177/apple-iphone-4-8gb-zwart.html.
As you can see, it's still actively being sold and also bought. Having it not being officially supported makes it become more like how Androids are not up-to-date.