Android dominates the smartphone market in terms of the percentage that use the OS, beating out Apple and Microsoft with just under 80% of the market. It's the ubiquitous nature of Android, which runs on dozens of brands and different phones, that allows it to gain such a larger percentage of the market than its competitors.
The same logic is at work in the tablet market, where Android devices have for the first time sold more than the iPad for the year. According to research from Gartner, Android tablets captured 62 percent of the market while selling more than 121 million units in 2013.
The iPad saw its percentage drop from 52.8% in 2012 to just 36% in 2013 while moving 70 million units, and Microsoft doubled its percentage on the year (but still only maintains a 2.1% share).
Like the iPhone, it makes sense that the iPad can't carry iOS to market dominance. Android is run on many devices, a lot of which cost less than Apple's products, which are still sold at a premium. Logic dictates the more numerous and less expensive products should control the majority of the market, and that's what happened here.
It's actually a testament to Apple that it took so long for this to occur, because it was more or less inevitable without a significant iPad price drop or a cheaper alternative from Apple. The more expensive iPad probably means more profit as well- cheaper Android tablets are likely not netting the manufacturers who make them as much profit.