1) a still growing cell phone market 2) a rapidly growing Android market
As long as the market's still growing, and as long as the vast majority of Android users aren't disgruntled about being left behind, their efforts are best spent on building new phones and acquiring new customers.
I'm curious how many people purchasing a new Android or iOS device even know it can be upgraded. Apple has features in iTunes and now in the phones themselves to advertise new versions and strongly encourage users to upgrade. I'd wager most Android users, because their phones never advertise updates to them, are aware it can even be upgraded and, thus, aren't the least bit put off by these vendors apparent negligence.
Until the market stops growing, or until the vast majority of users get angry about being left behind, vendors like Motorola have zero incentive to spend time or capital on maintaining old devices.