If you are still waiting, it may be that Microsoft has found issues with regards to the hardware in your system (drivers for a certain device under Windows 10 may not be available, or may cause critical issues with the system if you try to use them). They have a hardware matrix set up and are only pushing the upgrade notification to those who have systems containing hardware that has the best chance of a successful move to 10.
As the kinks get worked out of existing drivers and hardware sets, more people will get the notification to make the jump. As USAFRet said above, you can always grab a copy now using the link he provided, but you do so at your own risk. In that case, make sure you are prepared to deal with any issues that arise...