Peas freeze quickly because they are small. And, because they are small, so are the crystals that form when the water in their taut cells (imagine many minuscule balloons) turns to ice. And small crystals cause far less damage, puncturing and ripping to cell walls, which means that, when the pea defrosts and the ice returns to water, the cell walls are able to hold it, resulting in a firmer pea.
Larger items, on the other hand, see the formation of larger ice crystals, which can puncture the walls (again, imagine minuscule balloons, but this time with lots of tiny rips), making them leaky when they defrost and causing mushy vegetables. This is why peas survive the industrial freezing process so much better than, say, carrots or a slice of aubergine.
