Apple had a fantastic quarter, selling 61 million iPhones between January and March of this year. But one huge weak spot was the iPad. Not only were iPad sales for the most recent quarter lower than they had been a year earlier, but those sales, in turn, were lower than the sales in the first three months of the 2013 calendar year.
As you can see, the latest results are part of a broader trend. After four years of growth, iPad sales peaked in the 2013 holiday season (Apple's fiscal year starts in October, so the company calls this Q1 2014), and have been declining ever since.
Why is this happening? I suspect two factors are contributing to the iPad's decline. First, people who already have iPads just don't have that much reason to upgrade. People don't take their iPads around the house as much as they do their iPhones, so there's less risk of damaging them. And the iPad hasn't shown big leaps in performance in recent years, so people who already have an iPad see little reason to buy a new one.
The iPad may also be hurt by the trend toward larger iPhones. There's not a huge difference between the 5.5-inch display on an iPhone 6 Plus and the 7.9-inch display on an iPad Mini. So someone who in the past might have bought both an iPhone and an iPad might now be satisfied to just buy an iPhone.