I've seen comments around stating: "zero velocity at zero altitude", but no explicit reasons why. My uneducated guess would be that hovering requires more fuel than they want left in the rocket.

EDIT: Nope, not a fuel problem -- hovering is apparently out because with an almost empty first stage, the thrust/weight ratio of the engine is greater than 1, even with the engine throttled down as much as possible. So the engine can decelerate the stage, but can't hold velocity at 0, meaning the only way to land it is to time it so that the velocity hits 0 at the point of touchdown (hence "zero velocity at zero altitude").

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Musk has confirmed that the issue this time was a sticky valve: https://twitter.com/elonmusk/status/589577558942822400

"Cause of hard rocket landing confirmed as due to slower than expected throttle valve response. Next attempt in 2 months."