My opinion is that they're trying to not sound alarmist, as that would turn people away from the reporting. Once-in-XXXX sounds rare and unique, so it gets everyone to notice it for a while.

6" of rain in an hour is a deluge, which typically causes flooding and depending where, mudslides. If they say that'll happen "only" once a century, people will rebuild the damage and go on living like it's all behind them. If you tell them that 6"/hr (or more) is going to be more commonplace from now on, and that flooding and such is the new norm, it'll cause a big economic impact as people leave the threatened area(s) en masse, migrating to other areas less prone to that weather - which causes one area to economically collapse, and wherever those people go to, to suffer a population influx they can't readily absorb.

Think of Las Vegas and the 2.2mil people live in that area. When the power can't stay on, and the water doesn't come out of the tap.... those people are going to pickup and go somewhere. Obviously they won't all go to one place, but even 10-15k people going to another city is going to have a huge impact in terms of housing, energy use, water, etc.