Timex had the contact to assemble the ZX81 in Dundee (Scotland). I imagine that that made them the obvious choice for a US expansion.
Some background here [1] - this whole website has lots of nuggets of info if you’re interested in the early Timex / Sinclair machines.
[1] https://www.timexsinclair.com/blog/a-piece-of-cake-in-dundee...