Just a couple of ideas. Since these are both semi-offroad bikes, a triple might be just thought of as "standard equipment". I also think if you consider the entire US market, bikes with triples probably outsell bikes doubles by a large amount. So in the end, low to mid range triples might be actually be cheaper for the manufacturers than doubles.

Finally, the bike consumer on the entry level end of things might see more speeds as better and so it could be that doubles on the lower end models just don't sell as well.

I agree it would be nice to have the choice and maybe an LBS wouldn't mind swapping it out for you.