Because it's not cost effective. Or time effective. Or really any good reason to do it that way. I suspect the number of required reflectors and shaped charges needed would be sufficent to build several weapons with WGP. From that angle, it makes no sense to build an implosion with 20% enriched.I still have never heard of a successful implosion device that would work with 20%.
Because? With sufficent quanities of 20% (and I'm meaning LOTS) it shouldn't be impossible to compress it to the point where self sustaining reactions occur. I don't think it would work with the normal amount of material used in enriched. But with sufficent amounts 20% and lots of shaped charges and reflectors, it should theoretically work. But at that point, deployment as a weapon is virtually out of the question. When your weapon is as big as a house, it's not really useful. But that does not make it impossible.as far as I know it isn't even theoretically possible to compress the pit enough to allow the rapid disassembly of the device to become self sustaining.