That seems rather strange, given that you can upgrade your iMac to El Capitan. But, it looks like Boot Camp can only run the (64 bit) version of Windows 10 on the Macs shown in this link:
https://support.apple.com/en-us/HT204990
Unfortunately, your machine is not on that list.
Now, it looks like from this link:
http://www.parallels.com/products/desktop/resources/
that you can use Parallels with the 64 bit version of Windows 10 on your iMac, as according to the specs of your iMac:
http://www.everymac.com/systems/app...inch-aluminum-mid-2011-thunderbolt-specs.html
it does not have one of those unsupported processors mentioned in the Parallels link.
Note that the Parallels link does state that you can also use the 32 bit version of Windows 10. I am certainly no expert with all this, but that should imply that you could use the 32 bit version of Windows 10 with Boot Camp. But, this says otherwise:
http://www.redmondpie.com/boot-camp-6-for-mac-os-x-with-windows-10-support-released/