I think the mentality comes down to this capping esoterics in the most time efficient manner is tied to doing dailies.

Asking the developers to code in an accumulative cap is a bit tricky because you are in a sense asking them to compensate for a small variety of playerbase which would be those with an extremely erratic schedule. Ask yourself this if you have an accumulative cap compared to a weekly cap, how much more time is it going to take to fulfil the accumulative cap.

We'll say yes 7 days= 75(daily) otherwise it's 40 a run.

So to get to 450 it takes 6 runs, you have one left over, so you could say ya that's added to the "accumulative" 900 so now you are 525/900 meaning you still need 375. Now only getting 40 a run you have to run at least 10 dungeons to gain that.

Now let's look at it again, If you can just log on a day you only need to do 6 dungeon runs to cap esoteric, if you can find time each day just to play you only spend maybe 6hrs capping and even then as you are waiting for queue you can do other things while waiting, just stay alert.

However now we assume 1hr per dungeon give take queue time, let's go with 45 mins and 1hr so after those 7 days if you were able to make use of those dailies and still have 375 esoterics to get then you could still need to spend an additional 8 and half-10hrs of additional dungeons just to cap to 900 for a total of 14 and a half-16hrs spent playing in that one week compared to the potential of 6hrs spent each week, for a total of 12hrs capping in that manner.

I'm certain they have considered the idea of accumulative "cap" but when you notice the increased time it would take to cap in that manner they may have thought it wasn't worth the time to implement because the use of such a system is very very minimal at best.