I mean, there is no definite answer but I can suggest The fact is that the morning prayer has a preceding tahajjud prayer which is very long Already those who say tahajjud prayer are almost exhausted and Hazrat Muhammad Mustafa used to lie down on one side for a while and rest then said two rakat of sunnah but very lightly The sahaba say that it was the lightest of prayers that they had ever witnessed him saying and then the morning prayer was just a light prayer but not very light the prayer itself was not very light only the rakat were two because there were some others as well.
who hadn’t said tahajjud prayer and they wanted to start the day so for them the rakat in themselves were rather elongated in the sense that long surahs were recited in those rakat but after there was no sunnah nor was it permitted to say anything between morning prayer and the samra because that was in fact the time for the people to go out for work and in the modern society the timings are different I mean we can’t say the modern society because everywhere the things are different still the masses of people the great majority of mankind has a habit of rising early only the so called modern people have changed their habits to rising late so that still applies to the majority of mankind that the morning time is also the work time.
So after that they are made to go out and in asad prayer also the same philosophy seems to be working because after asad generally people go out for either walks or play games etc so it is prohibited to say any sunnah or nafal between asad and the sunset and that time is also left free so that people must go out and participate in other activities and also asad is very light although rakat the number of rakat it is twice as much as the morning prayer but because there are no long recitations from the holy quran and it is a quiet prayer set in heart so the time is even less than what you spend in the morning prayer similarly if you exercise your own mind you could come out with some more suggestions as to why but we cannot pin point this was the reason because we don’t know but by exercising one’s mind one can come out with suggestions enough as illustration
