With today's technology, why can't we find crashed planes
Having a technology and using it are two different things. As already noted, you are able to get pictures from Pluto because your tracking station knows where it is.

Similarly, the aircraft position is determined by Primary Radar, which literally scans the sky, but its effective range is limited and they are costly to operate. The secondary Radar locates the aircraft by interrogating its transponder, which provides the ATC with its location and speed. However, if the transponder is not working, there is no way for to detect the aircraft locations.

The aircraft can also communicate with the ground station using ACARS, but it depends on on board electrical system. The same goes for the ADS-B, which depends on onboard systems for determining its location and speed.

Its a bit like trying to find your mobile after switching it off. No app is going to work.

Assuming that the aircraft is involved in a crash, the Emergency Locator Transmitter (ELT) can be used to trace the aircraft. However, these are not mandatory in scheduled flights(though ULB is mandatory).

Satellites could help, but again, they should know where to look for.

The wifi or working mobile phones onboard could help, but these are yet to be implemented in any big way.

The next generation technologies under development will allow  real time tracking of the aircraft, but these are still under development.