Socket pins can be made much more compact than package pins, and hence higher pin-counts can be achieved using LGA. For the same reason, AMD uses LGA and not PGA for all its packages with over 1000 pins. Intel switched to LGA from PGA because it wanted to maintain a package size similar to s478 package, while making room for 775 pins. LGA775 packages ended up being more compact than AMD's PGAs.
AMD has LGA1207 and LGA1974 (G32), which are both LGA, Opteron processors use them.
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So the debate between PGA and LGA cannot be reduced to Intel vs. AMD by angry AMD fankids. PGA is archaic whichever way you look at it. It poses pin-density limitations, in turn pin-count limitations if you don't want to enlarge your package dimensions like no tomorrow.
If AMD wants more pins, for more memory channels, more PCIe lanes from the CPU, or simply more HyperTransport links, the transition to LGA is inevitable. Opteron products already made that switch five years ago.