Tablet ownership has increased
Tablets, when compared to other computing devices like Laptops and Desktops, cost a lot less. They do not necessarily have to have subscription service for data, unless you want to have 3G or 4G service supporting it (Smartphone and Tablet plans range anywhere from $35 to over $100 a month, depending on carrier and coverage). So, when the economy retracted between 2007 and 2009, and the recovery was not as robust as many of us have hoped, users were faced with some stark financial choices. They were also compelled to adjust their entertainment dollars as well.
That was the financial side of things. Now, if you look at the functionality, many users just want to check email, respond to social network updates and notifications, and check the news. Tablets are great at this and have battery life far better than most laptops. Apps have also made tablet use more productive and allowed some users to avoid their laptops and desktops unless there is a task that is just more efficient to do on a laptop or desktop. Tablets and smartphones, because of this increased functionality and productivity at a lower initial cost, are probably the biggest reasons that the PC market is in decline (not Windows 8, although some cite it as a reason).
So, it is not a huge surprise that tablet ownership has increased.