The answer to the question before I even say what the question is, is….they do it to supposedly make a smoother line or"to look better". 
The question is asking why do women’s undergarment makers such as Maidenform, Playtex, Victoria’s Whatchamacallit, and so on, make the straps so that they can only be adjusted in the back!?  OK.  So you make a mental note, some sort of approximation, of where they should be either lengthened or shortened to.  There is noting left to do but put the thing on.  Most times, it still needs an adjustment., at least for me it does.  So..then you have to take the thing off and try again.  You may or may not have it right.  Chances are that since it is such a pain in the neck what with taking it off and putting it back on, you just give up and try to stand wherever its adjusted to rather than go through that whole process again.  Then, too frequently, you have the "strap is too short" to begin with syndrome going on.  The only adjustment needed there is 6 inches more of strap on both sides so that  you don’t feel like you have just been garrotted!!! 
My rant is we who don’t care for the back adjustment dealie, are sometimes faced with too short a strap to begin with (are we all munchkins I ask you?) and the adjustment is most times in the back.  Two irritating things for me as a consumer.  I am not a large woman in weight so it’s not a question of being too heavy. The straps are too short in some cases. 
Bring back the front -of -the- garment  strap adjustment for pity sakes.  Bali has a few models like that. now.  Looking back, I don’t believe that there was a neon sign on most women  flashing the message "My bra straps adjust in the front,  Woo Woo" as they went about their day… making it imperative that manufacturers change that societal mishap immediately!  Manufactures would have been more customer friendly if they had found a way to survey first before they inflicted that on us.
Know what?  Either the treadmill or the frosty outside is waiting for me.  I am as always trying to spin out that time to when I must "just do it."
Oh and in answer to Loach as to how do you get someone to cook for you?  Charm, my dear…charm.