If you installed the Insider build clean, meaning NOT over an existing Win7/8/8.1 installation, it activated back then because MS used generic codes for their Insiders.  But now, those codes have been discontinued. 

That kind of Insider installation can not be activated anymore, and if you try to use an existing Win7/8/8.1key, MS will reject it — because they don’t allow the use of those keys to Activate Win10.

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