>>37430704golem/machamp is real but I think the trade requirement for alakazam/gengar was an afterthought when they realized it'd be weird if only two pokemon got to trade evolve, and they wanted to keep it to 3-stage lines (because the second stage is usable if you have no friends). This left:
>Butterfree/Beedril (literally useless until you evolve so this wouldn't work)>Pidgeot (often someone's first catch, more sentimental - don't want to trade it)>Nidoking/Nidoqueen (moon stone - ground-type wouldn't make sense with just trading)>Vileplume/Victreebel (leaf stone - muh elements)>Poliwrath (water stone - muh elements)An argument could be made that Sabrina ties the Gengar and Alakazam lines together, but that's very anime-motivated. If abra was available around Lavender Town I'd believe it more, but they wanted you to see your rival's Abra->Kadabra and have Kadabra by the S.S. Anne fight so they put Abra up above Cerulean.
Abra/Kadabra make more sense as a cave-dwelling pokemon and should've been available in all the caves except diglett tunnelI remember reading somewhere once that Gengar was originally intended to be a separate ghost-type from gastly/haunter that they combined at some point in development (which is why it gets a physical form out of nowhere), but I don't remember the source. This also adds some weight to the gengar as clefable's shadow theory although I think that's really just lazy design in G1 where unique silhouettes weren't valued during the design process
