"I wonder why the impact of Lunar Prospector, which took place ten years ago this July 31, wasn't seen from Earth?

Was it just the location or the less advanced observing technology, or a combination of both?"

The lunar prospector was small inexpensive orbiter which fully fueled weighed 296 kg.
http://en.wikipedia.org/wiki/Lunar_Prospector

Kaguya was nearly 10 times more massive.
"The "KAGUYA" was conclude its scientific mission to the Moon through a uncontrolled impact on the lunar surface.

Impact date: 18:25, June 10, 2009 (GMT) Near side, night time area
Impact location: E80.4, S65.5"
http://www.spaceref.com/news/viewsr.html?pid=31483

So Kaguya was more massive, hit in the night area of the near side around the latitude on earth of Alaska/Norway.

Lunar prospector used up all it's fuel and targeted and hit the shadowed area of the moon. The purpose of the impactor was to look at plume from the impact- which was done, but no water was detected in the plume.

NASA's orbiter, LRO, LCROSS launching next week will attempt to impact a mass similar to Kaguya in a dark crater and will monitor the impact with a small spacecraft which will follow it down and which will then also impact, leaving the main orbiter which could also monitor the impacts and continue with other science objectives.