Because Samsung didn't build it for that device.
That's the simple answer. To expand on that, there are several layers to an OS update - support by the SoC manufacturer for the new OS, desire of the device manufacturer to build an update for the device, performance of the update once it's tested (if it's not good it won't release). Samsung first needs Qualcomm to provide support for the new OS in order to even begin developing an update, this is one reason Google decided to start building their own chips.
All that said - there are some VERY smart folks over on the XDA forums that surely have found a way to shoehorn newer versions of Android onto the device through some crafty reverse-engineering. Doing that is not without risk, and isn't for novices...
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