Service and Fraternal organizations have been declining since the early 60's. The WWII generation joined them in droves and swole the numbers up tremendously, but then their kids did not join. So as the WWII generation dies off at an incredible rate, there isn't the next generation to take their place.
In the Masonic groups the WWII generations grandchildren and great grand children are now starting to join in significant numbers, so the decline in membership is slowing.
It really was mostly about a societal change though, people quit joining groups, churches, service groups, PTA's, churches even bowling leagues, you name it, they are all feeling the same pain.
If your interested there is a great book called "Bowling Alone" which studies the situation.
Another possiblity as well is the building simply became more than they could afford with upkeep and maintance and they are simply moving on to a new location as well.