Netflix says: "The hurdle has been the lack of a generic and complete platform security and content protection mechanism available for Android." They can't get Netflix on Android because they can't get a secure DRM system across Android devices.
According to Greg Peters, from Netflix product development, the difficulty in securing a common DRM system on Android devices is "the same security issues that have led to piracy concerns on the Android platform." He goes on:
Setting aside the debate around the value of content protection and DRM, they are requirements we must fulfill in order to obtain content from major studios for our subscribers to enjoy. Although we don't have a common platform security mechanism and DRM, we are able to work with individual handset manufacturers to add content protection to their devices.
By dealing with individual manufacturers on a phone-by-phone basis, it means not every Android phone will get to have Netflix. Which means that F-word comes up again.
Unfortunately, this is a much slower approach and leads to a fragmented experience on Android, in which some handsets will have access to Netflix and others won't.
To Netflix's credit, they don't want to cause confusion among their customers but smartly believe that having some Android phones have Netflix is better than no Android phones having Netflix. In any case, those Android devices that will get Netflix should expect it early next year. [Netflix Blog]