We have a list of devices that will get free update to iOS 10 at fall:
iPhone 6s
iPhone 6s Plus
iPhone 6
iPhone 6 Plus
iPhone SE
iPhone 5S
iPhone 5C
iPhone 5
iPad Pro 9.7-inch
iPad Air 2
iPad Air
iPad 4th generation
iPad Mini 4
iPad Mini 3
iPad Mini 2
iPod Touch 6th generation
Those models will not get update: iPad with Retina Display (3rd Generation), iPad Mini, iPod Touch 5th Generation, iPhone 4S and older models.
One reason is performance - it's possible they would work to slow or have problems with dupport of some new features. But there is second reason:
Those are old models so from Apples interests view it's better to stop support for then and encourage users to upgrade their devices.
Source: CNET