Moshe was given a series of three signs (previously mentioned as tricks, or magic tricks) with which to convince the people of Israel to trust in him and follow him. First he would use one sign, and of they still do not believe then the second, then the third.
If the third sign was the mother load and that was so strong it would do the job the first two could not do, why not just use the third sign right away? Why bother with the first two at all?
You never reveal your best hand until it is necessary. Sign #3 represented a very powerful message. It carried a very strong threat to the Egyptians and represented a very bold move by the Hebrews. You do not want to give that away unless it is absolutely necessary.
First try other things, then if it is still necessary, go for the nuclear attack.