NOAA's data. 20% fewer data points?

I'm still plugging away at NOAAs data. I have not yet come to a point that I'm happy with the data yet, but that is mostly because it's a terrible export. It is obviously from a database, but the format is indefensible. My respect for data in the soft sciences has waned considerably.

I have come across something that is odd, but not necessarily wrong. The number of weather stations trends up, and then down. I'm not sure what to think about this yet, but there are about 20% fewer stations today than in 1990.

Please excuse the huge graphs, but it's clearer this way. Why would one remove stations if the data is valid?