Although there have been some attempts by companies like Asus and HP to make some Android hybrids with detachable keyboards, we've never really seen a big wave of Android notebooks coming to market, and my guess is that's mostly Google's fault, and not the fault of OEM's.
First off, it's understandable why OEM's wouldn't want to push Android notebooks into the market right now. Not only is Android not fully optimized for such a form factor, but many of its apps aren't even optimized for 10" tablets yet, let alone 12" or 13", or even bigger laptops and screens.
Before OEM's will even consider putting Android on such machines, Google will need to optimize Android for them, and allow for more advanced multi-tasking, for starters, and use an UI that makes more sense on such a large screen. Blackberry's QNX/BB10, Samsung, and even the Youtube team, has shown us that it's possible to have apps fully running in the background (not "paused", as it happens with most apps on Android now, except for some background processes), in a way that doesn't completely kill your battery within minutes. Of course, multi-pane/window capability would be a must, too, for an OS that's supposed to make you productive.
Google has taken so much time to launch the Android 5.0 version that I've got to believe they are preparing a new multi-tasking system for Android, too. But this version probably won't launch until summer, or even next fall, if we get another "surprise" from Google, and we only get to see Android 4.5 in summer, with the new Nexus 7/8, and Android 5.0 with the Nexus 6 (or Nexus 5 2014, because I certainly don't want them to make a 6" Nexus phone).
But the real question is if Google is even working on such a multi-tasking system right now, and if they are really working to prepare Android for notebooks. Perhaps they just don't want Android to be used on notebooks, because they want only ChromeOS available there. I think that would be a big mistake on Google's part. If they want to beat Microsoft and Windows, then they need to hit them fast, and hit them hard with Android.
Android and iOS are the ones eating Microsoft's lunch now, not Mac OS X and Chrome OS. So why not take full advantage of that, and create an Android OS for notebooks that could be taken seriously as a "work OS", too, by many people (granted the apps for that are there, too), instead of slowing trying to move the needle with ChromeOS? OEM's want to be able to do that, because they want a serious alternative to Windows in the PC market, and I think users would want it, too, especially if it's executed well.
There's no reason Android couldn't become a serious OS alternative in the PC space, if it's properly optimized, and developers start writing the kind of apps you'd need on a PC. Next year we're also getting 64-bit ARMv8 "desktop-class" processors (as Apple likes to call them), and a move to the 20nm process, which means even more powerful and higher frequency chips for the same or lower power consumption.
These new chips combined with a more advanced Android 5.0 OS could be the "perfect storm" for a wave of Android notebooks. It's time for Android to grow up, and move from being a smartphone and tablet "toy OS", to being a full-fledged work OS, that you can do anything on.