It has to do with drivers and how the software is written. Android has a whole lot less "universal drivers" than windows, and if the driver is not up to date enough to work with the OS you can't update.
Second not all androids share the same layout for things like the bootloader.
Thirdly there is not one version of Android. There is an original version of android which is called the AOSP and their are different versions of the AOSP (4.0, 4.1, 4.2, etc). All the oems such as Samsung, Dell, HTC, etc actually modify the OS, the AOSP and take features out and put futures in.
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Android is not one "species" but instead is more of a "family." In biology (bringing up biology to use an analogy showing how they are similar) if you can mate with one another you are the same species, above species is "genius" and above genius is the term "family." For example the
- Grey Wolf also known as C Lupus is a Species,
Wolves are part of the Genus Canis which includes dogs, wolves, jackals, and coyotes
- The Genus Canis, aka all those animals are also part of the Family called Canidae family which now includes foxes.
- Canidae are part of the Sub order Caniformia.
- Caniformia is part of the Order Carnivora
Even though a hyena looks like a dog/wolf/canine it has very little genetically in common with a canine. Hyenas are part of the sub order Feliformia which is the same order that felines come from like Lions and Housecats.
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Now windows on the other hand is windows. Every dell, hp, gateway, computer you built yourself runs the same OS. Sure the OS is far more bloated for there are parts that they do not need, and all these extra drivers your computer will never use but hey you can put a computer you bought 10 years ago in 2003 with XP, Vista, 7, 8, 8.1 and probably windows 9 on it.
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There are advantages and disadvantages to what microsoft and google is doing. The disadvantage of what google is doing is that it is harder to update the software. Google is trying to do certain things to remedy this such as a common platforms / reference designs such as
Android One, and allowing a lot of things to be modular so you can use
google play services to update things such as security and other features of the OS. With new devices such as android wear, android tv, and android car it appears OEMS will have limited to abilities to skin the device thus there is a greater chance for updates since then the only thing holding it back is drivers.