I've read through that article, and there are several mistakes in it. I'll answer your question first, and then point out the errors in the article at the end of my answer. :)
Depending on the frequency, microwaves are strongly absorbed by water and can also be absorbed by oxygen. This is because microwaves can induce several rotational modes of vibration in a water molecule. This is why microwaves are used for cooking as well. But the consequence is that if you point your microwave emitter at hail/ice/water, a large fraction of the emission will be absorbed.
Radio waves are made of lower energy photons than microwaves (since the energy is related to frequency by $E = h \nu$) and aren't easily absorbed or scattered by anything in the lower atmosphere, which makes them a good candidate to send and receive signals over large distances. They are scattered by the charged particles in the ionosphere, and this effect is used in radio communications. But there are no planes in the ionosphere, so this effect doesn't have to be accounted for in radars. :)
Now for my nitpicks.
Firstly - they mention a magnetron which emits microwave radiation travelling with
a speed almost equivalent to light waves but with longer wavelengths and higher frequencies
which is rubbish. It's moving at exactly the frequency of light, since it is made up of photons.
They also say
As radio waves travel at light speed, they are faster than most of the objects.
Which is also rubbish. They travel faster than all of the objects. Nothing can travel at the speed of light. These may seem like nitpicks, but for a "scientific" article, I think it's important to get these details light.