There is no point in rebooking pax - nor it is required under law - when a departure is postponed from 1-2AM to 6-8AM. First of all there close to none departures with other airlines from IST during these hours, secondly you gain nothing (i.e. earlier arrival at final destination) by rerouting.

Whilst these systematic delays are annoying, requiring TK to rebook OAL in these situation, has no merits/doesn't make sense. Secondly you will keep hitting your head a against a brick wall when trying to accomplish a rebooking. If the flight is canceled, it is completely different.