Demand and added cost. In the USA only two major networks offer SIMs (TMobile and AT&T). The other two including the giant Verizon, and Sprint don't offer SIMs. Thus phones designed for this market either have 1 SIM slot or none at all!

Then take into account European and Asian markets (and other countries like Africa) where they do have providers with SIM slots, most customers will not change SIMs often enough to warrant having two. Unless you were an employee whose company pays for work related calls and nothing else or they travel often enough where switching SIMs would be annoying (e.g. each day you travel say between Germany and Italy) or something.