Phones and Tablets are different problems - with phones and 3G/4G/LTE tablets, you've got a carrier who can push updates to you, but if you've got a Wifi-only tablet, there's no carrier, just a manufacturer. Do they have an incentive to upgrade? Does the user have a way to tell?
Google's new product announcements always say "See all our shiny new features! If you have one of these three Google Nexus products, you can get it! Otherwise, wait for your carrier to maybe do something!", but never say (at least to consumers; I assume they tell manufacturers) "If your device has at least this generation processor and this much memory, you can upgrade, here's how." Part of that is because, for the big-vendor phones, the manufacturer and sometimes the carrier heavily customize the product, replace half the user interface and tools with custom ones and add a bunch of useful apps or bloatware, and then you can't just do the OS upgrade yourself because you'd lose the customization and probably also lose the bloatware.
My old HTC phone was heavily customized, and the upgrade from 2.1 to 2.2 wasn't actually pushed out, though you could pull it for a little while, if your phone wasn't broken when locked-to-AndroidMarket got replaced with Google Play. My noname 4.0.x tablet which has Google Play but no obvious customization is now running 4.0.4 (I think it originally had 4.0.1), so it shouldn't be a problem to upgrade it if it's got enough horsepower - and Google never tells you how much horsepower they need, just what Nexus models support it.. I ended up replacing the HTC with a Samsung, and haven't taken the time to go back and install Cyanogen on the HTC; I assume if I did that to the tablet I'd lose Google Play access, which I depend on for apps and patches.