If I were to hazard a guess, I'd say it's because production in a gas power plant is simply regulated by burning more or less gas. If you need more gas, you can likely buy more. While production in a hydro power plant can be brought up and down by draining the reservoir faster or slower, a hydro plant has a lot of external factors that a gas power plant hasn't.

The current water level in the reservoir, expected drainage and rainfall over the year, environmental regulations concerning water discharge and intake levels, fish, etc. all play into it. Note that a hydroelectric power plant doesn't just generate electricity, it also regulates water levels pretty far up- and downstream from the plant.