It's because Vivec traditionally was based on Vvardenfell, and thus there's a city bearing his name there. Sotha Sil, on the other hand, is a mystic and scholar, who spends a lot of time in his Clockwork City. It's not impossible you might encounter them sometime during the course of the game - but you can't really demand to encounter every single character within the actually quite small areas available to travel in. Half of Morrowind or more remains inaccessible, so...

bigger question is... Where is the city of Almalexia?

but yeah, vivec is having fun levitating in his little room on Vvardenfell(well, he may be up and about at this time in history), while sotha sil is busy too playing/studying... stuff, presumably dwemer in nature, to bother with us mortals.