most likely this is because of the market wants (i.e buys)
here is my speculation, not fact, speculation
105/ultegra/dura ace are road groups so most buyer need or think they need the 50...and the 34 when combined with a 34 max rear cog pretty reasonable 1 to 1 low gearing
46/36 could be either for people trying to run some thing like a 10/42 (if they can get a rear derailler to work with that which is a question)
46/30 is logical because it give lower range and keeps with the current shimano rear derailler spec no more than 16to difference in crank rings
I really don't see 46/36 but similarly I don't see much need for 46/34 it gives a bit more low than 46/36 but has same high end limitations
maybe search shimanot press releases?
__________________
Life is too short not to ride the best bike you have, as much as you can
(looking for Torpado Super light frame/fork or for Raleigh International frame fork 58cm)