To earn more money.
Hmm. Sure, this way they may reach many customers who otherwise might not consider to venture into this field.
On the other hand, they scare away the quality-minded users.
Every such user with some basic knowledge - not at least with regard to the antenna and wave propagation - should have comprehended: We need an outside antenna, for optimum link quality, ditto range, and radiation protection. Also, we want to have a decent surfing experience, which means we should have at least an 8" monitor.
The solution formula results from these demands: You need a netbook connected to your cellphone's HSPA modem; the netbook stands in your trunk and is being controlled through a near-the-steeringwheel periphery; the cellphone lies in a cradle which is wired properly (car battery, mic/speaker, and most of all --> roof antenna <--).
This way, you have a professional formula, and standardised with your home/office computer (same OS, same software as on your usual computer, instead of many more through a smartphone - many more to get used to, to worry about). No smartphone ever will beat this formula.
Why don't they offer BOTH formulas to choose from? This exclusive insistence on smartphones makes me think the whole thing is due to government intervention: They don't want people to be on equal footing with law enforcement. So if this theory is true, then the smartphone phenomenon is not a mere economic decision.