When driving in rain, the road becomes the most slippery right after it starts to rain.
As the first rainwater hits the road, it mixes with the oil, rubber tire particles, and other gunk on the pavement, forming a very slippery coating on the surface.
After it has rained for a while, a lot of that gunk gets washed away, and most of the slipperiness is just water ... which is pretty slippery on its own, but not as slippery as it is when it has oil and rubber particles in it too.