weak cervix when cerclage?

okay so I have a history of weak cervix I started having contractions last week at 22 weeks. they did a cervix check and I am shortening not dilated or anything yet but at 2.8 instead of 5. now ive been on progesterone shots since 16 weeks. the only thing they said was they will check it in 2 weeks and if I have anymore labor signs to come straight in. what I dont understand is why they arent putting a cerclage in immediately and just waiting?? ive never had one before my last just cane at 30 weeks because I dilated to a 5 within hours at 28 weeks.