Why they lost cantact with NASA's Mars Rover

Beagle was on a cut price budget and working to a tight schedule. Though I reckon it was just sheer bad luck it failed.
NASA does have the money and the technology but it must be remembered that one sharp rock at the landing site could have scuppered them too.
Landing on Mars is a difficult job whatever amount of technology you use.

As for the suggestion that aliens got to the rover. I reckon their best tactic would have been just to turn the bugger over. Ever seen a turtle on it's back?