I think it's because that AdSense revenue is so sweet and so easy that nothing else is as interesting to the organization. Pretty difficult benchmark for competing products to show their value. It's like an oil rich nation investing in unrelated industries for which they have no history or pedigree in order to build a sustainable future.
There's also a well-documented culture within Google that it's an attractive resume-builder to launch new products, but nobody wants to maintain a product.
