Simple: nature of chakra.
Kakashi had a tough time to learn it since he was a lighting type, he said it himself that doing it is close to a miracle.
Sasuke with fire have a lighting type too. So the rasengan is something he can't master without spending a shit load of time. Moreover sasuke never had any master who knew it. There is only 3 people who knows rasengan at shipuuden: Naruto, jiraya, kakashi.
So who's gonna teach him ? Kakashi knew it because he was the 4th pupill and a friend of Jiraya.
Moreover naruto would have a better control of chakra than sasuke since all his training since early shipuuden is chakra control:
Rasenshuriken: chakra control+chakra nature
Hermit mode: pure chakra control of 3 different energy.
Bijuudama: chakra control of two opposite energy and combine them.
Why sakura didn't learned it ? Because she doesn't f*** need it. She is a medic, all her fighting skills are meant for defense since she have to stay alive. Learning a offensive ability is not useful for her. Can she learn it ? She can but i think only stage one.
The stage two need a skill that even the allmighty 4th couldn't achieve in his lifetime. Naruto could because of shadow clones specs, but he is the only one who can do it.
the question of the topic is not: Sakura will have the seal ?
It's can she have it ?*
And yes, she can, of course. She will have it, since she is a younger clone of tsunade in ninjutsu abilities. It's just the timing, and how it's done that sukcs and looks like an asspull. She should have it done sooner or after the war, not when the trio was together. Even if it was destined by plot to activate here, then she could had just said that she needs to save chakra for a later use, it would have been a hint.
But there is none.