The answer is power generation. Any ground-based shield will have more power available than a starship. The shield technology is the same, but ground-based stations can use more (and more varied) power supplies, and don't have to worry about propelling the generator, its fuel, and armor/weaponry around.
The Star Destroyers could have battered down Echo Base's shield - there's no question. That's the purpose of the Imperial Star Destroyer - they were designed to batter down planetary (or area) shields and damage planetary installations (in addition to their role in space combat, of course). The shield was only a delaying tactic. It prevented the Empire from destroying the base from orbit in a few shots. Destroying the base from orbit would have required them to sustain a bombardment for a significant period (how long is unknown, but certainly longer than the ground assault took).
The point of the ground assault was speed - they wanted to seize the base with as much data and material intact as possible, and capture as many Rebels as possible. They wanted to force the Rebels to lose valuable resources or even more valuable personnel in an attempt to evacuate those resources.
The shield (and Rouge Squadron's snowspeeder hampering maneuvers) bought the Rebellion time to load the transports and escape aboard them. Had the Empire taken enough time to batter down the shield, the Rebels would have had enough time to load more of their heavy equipment (which you can hear Luke ordering abandoned in ESB).
By rushing the base, Vader and the Empire also hoped that the Rebellion, in their haste to escape, would fail to completely erase their databases and wipe or destroy their remaining equipment. The Empire had extremely skilled data analysts, and any bit of data they could recover would help pinpoint other Rebel bases/operatives/plans.