It is not difficult to excite people to lend a hand to a worthy cause. As soon as someone comes up with a new idea that sounds worthy and doable people are ready to lend a hand. Jews have always been in the forefront of new movement designed to help others.
The problem often exists where after the particular movement or project begins to drag on people lose interest and begin to slacken in their enthusiasm and the project begins to fall apart.
When the Mishkan was erected and the sacrifices were brought on the Mizbe’ach people were excited. It was something new and meaningful. Hashem knew in His ultimate wisdom that people will soon grow weary and the sacrifices will diminish and the flame will go out. Hence He instructed Moshe to forewarn them that the fire on the Altar was never allowed to go out.