On the face of it you're right, the Antonine Wall is indeed more defensible than Hadrian's. From the evidence of the number of forts and other defenses there seem to have been at least as many as stationed on Hadrian's Wall, but covering around half the distance. There were also significant improvements added to this structure, most notably a Military Way - a road that runs parallel with the wall, allowing lateral troop movement - a feature that was not added to Hadrian's Wall until it was reoccupied following the retreat from lowland Scotland.

So why was the Antonine Wall abandoned? It's a interesting question, but unfortunately not one we can give a fully secure answer to. First things first, I think I should clarify the chronology, just so everyone understands the order in which things were built.

AD 122 - Hadrian comes to Britain and commissions a wall "to divide the Britons from the barbarians" (SHA Hadrian 11.2). Construction begins.

AD 128 - Construction of the wall finished.

AD 128-138 - 12 forts added to the wall, all of which house auxiliary units tasked with manning the wall.

AD 139 - Antoninus Pius orders troops into Scotland, under the command of Lollius Urbicus (SHA Antoninus 5.4, RIB 1147). Hadrian's Wall abandoned.

AD 142/3 - Coins of A. Pius proclaim victory over Britain. Construction of the Antonine Wall begins.

AD 154 - construction completed.

AD 162 - Marcus Aurelius now emperor. Antonine Wall abandoned and Hadrian's Wall now reoccupied.

As you can see from this rough timeline, the Antonine Wall was really only used for a very short period of time - perhaps 8 years from completion to abandonment. There must have been some obvious reasons why it couldn't be maintained for it to be abandoned so quickly, but what those reasons are are unclear. There are suggestions that moving the frontier further north, and therefore closer to tribes more hostile to Roman rule, meant that the defenders were attacked more often. They were now considerably further from their supply lines and legionary reinforcements at York, so perhaps the Antonine Wall was too much of a stretch? The other possibility is that Marcus Aurelius removed a number of troops from Scotland to use in his campaigns in Germany, meaning that it would be impossible to hold the new frontier.

The other possibility is economic. On getting to this new northern frontier, the Romans may have realised that there was little to gain from taxing the inhabitants, who would've been relatively poor farmers. Considering the outlay of money needed to launch this campaign and build new fortifications, the Empire needed something back. If that wasn't forthcoming, why stay?

Sources: D. Breeze, ‘Roman Scotland during the reign of Antoninus Pius’ in Hanson and Keppie (ed.s), Roman Frontier Studies 1979 = BAR (International Series) 71

J. Gillam, J.P. and J.C.Mann, ‘The northern British frontier from Antoninus Pius to Caracalla’, Archaeologia Aeliana (4th series) 48

L. Keppie, ‘The building of the Antonine Wall: archaeological and epigraphic evidence. Proceedings of the Society of Antiquaries of Scotland 105 (1974)

M.Speidel, ‘The Chattan War, the Brigantian Revolt and the Loss of the Antonine Wall’, Britannia 28 (1987)