Marketing
I can't help but think that part of the problem is marketing. I am a happy owner of a Padfone 2 tablet/phone combo, and most people who see it have never heard of it. Unlike Samsung and Apple which are now household names, no-one seems to know who Asus are any more.
My Padfone is roughly equivalent to a Galaxy S3 spec-wise, the hardware looks and feels better built, and came with a dock to turn it into a 10" tablet, all for £5 less per month than an S3. They should have been selling like hotcakes, instead I found only two places in the UK actually selling the device, and they rarely had stock. How do Asus expect to sell anything if no-one is stocking their products?
The Transformer was one of, if not the first Android tablet with the Keyboard dock to give it a laptop form factor. The Padfone is still unique to Asus. They are outputting ideas that are different to the masses and make a lot of sense. Even the "normal" devices like the Nexus 7 are very good, solidly built pieces of kit that should compete favourably with most other manufacturers. It's a real shame not more people have cottoned onto what Asus have to offer.