What is the difference between a glider and a sailplane?
The sailplane article is focused around sport and recreational planes. The aircraft article describes a wide variety of aircraft, not simply planes, like hang gliding and paragliding.

Which makes sense - one is about aircraft, so it has the aircraft moniker. The other is specifically about planes, which are a subset of aircraft, and thus have the sailplane moniker.

There was an idea of merging them in 2011, but it failed.