It costs a lot more money to build a 12-14 story tower than a bunch of low rise buildings. You don't have to put a low rise building's foundation to bedrock.
I haven't looked at the old ground surveys, but its completely possible that the ground was found to not be suitable for a tower in 1970, but with new technology in the last 50 years for drilling/sinking pilings, they could build on the same place now. We know this is the case as they have returned to prior locations previously deemed 'unsuitable' to re-survey for future builds.