It may seem to be unnecessary as one hour of switching off your appliances wont make much difference. But a collective effect, where everyone does it together creates a butterfly effect, where it rises its effectiveness, rather than not having any earth hour. Eventhough little compared to other methods of saving the earth, at least effort is shown
No, we should not make Earth Hour compulsory, because people only celebrate Earth Hour for political reasons. It is unfair for people who want to literally return to the Dark Ages (Dark Ages, because there are no lights), to impose their sociopolitical agenda on everyone else. People who want to celebrate Earth Hour should do so, but should lead by example, not by compulsion.
The only way to make earth hour compulsory would be to impose it by law. That would be incredibly problematic to enforce and hurt the image of the program in the long run. Earth hour's primary goal is to spread awareness of lowering energy consumption and part of that requires people to do it without a penalty attached for not participating.