If an ocean has been discovered on Europa, why isn't NASA sending a probe to investigate it?

We do not know if Europa has a sub surface ocean still in existence so I wouldn't say that we have proof of an ocean on Europa. The fact that it has a magnetic field seems to mean that it probably does have such a liquid water interior, laced with some conducting ions like salt to form the currents that create the magnetism.

As for NASA, there are several proposals on the drawing board for a Europa mission, but these are costly and you had best make certain the data you have is telling you what you think it is before you invest in another billion-dollar probe.

One probe is a Europa radar mapper which can probe below the crust and search for liquid water. Another is a surface penetrator mission that would send a projectile to burn its way through the ice crust leaving a transmitter behind on the surface and reeling-out a cable as it goes. Once through the crust, it would eject maneuverable robots in the purported ocean to send back up the cable, information about water composition, temperature etc.