Answer? Because they have to train powerful ninjutsu other than learning medical ninjutsu.
Medical ninjutsu is not for everyone, I ask myself before why can't Hinata be like Sakura and Ino and be a medical nin? Because Hinata came from an exclusive/prestigious Hyuuga clan and has kekkei genkai, Byakugan... so why will she even bother to take her time to learn medical ninjutsu? When she can't even master the 8 trigams 64 palms? She needed to train more on her ninjutsu. Unless, she has more time to learn medical ninjutsu, to add it up to her abilities...
Same goes for Ino, who inherited the Yamanaka clan jutsu: Mind-transfer jutsu. She needed to train her skills into that, especially for the Ino-Shika-Cho formation. She just wanted to learn medical ninjutsu, because she felt weak at the time and thought about the rivalry to sakura.
Well, Sakura... is useless. She did not came from a powerful clan and does not have a kekkei genkai to begin with. But for those reasons and weaknesses she knew that she is smart and has strong will – enough reasons to be a medical nin. Especially, being good at controlling chakra (back at the time where Kakashi-sensei taught them chakra control, she is the first one to learn it), smart (seen at the chunin exam's first part), and has a strong will (being passionate to be of help to her comrades, save Sasuke and Naruto). It is the easiest and most sensible way for her.
That's what I think haha
