Because it has over 200 million of paid (credit card verified itunes) users, who have actually made a purchase off iTunes and are willing customers with a guarantee to buy for the right product( song). So music companies or app developers who want to sell to customers via targeted advertising, find Apple (iTunes) as the best online platform to actually generate a sale revenue for themselves, thereby enabling Apple to fatten its wallet share.
That is also exactly the reason why iOS is way more valued as a platform than Android.
Steve Jobs understands that the only loyalty in marketing is when you commit your money to the brand, not just for today , but even for tomorrow.
