Its been my experience that some people are just afraid of big trees in their yard, and even after explaining to them that its healthy, low risk, and providing all sorts of benefits to their home, they still want it gone.
There's also the chance that there was something wrong with the tree that wouldn't be noticed from a distance. If it had root rot, for example, it may have looked healthy in the top, but had fungi growing on the roots. With all the rain we had this winter, I've seen a little uptick in root rot. A tree with root rot usually looks healthy until the day it falls over.
I hate to see it happen, but in the end, its their property, and they get to make that choice.
Chances are, their power bill just went up, and their property value just went down. A nice, big tree can add a lot to the value of a home.