why the magicians not used their power properly

I guess this makes the story more interesting for the whole series. Saruman the white may have been corrupted by his greed to power and uses his own power to dominate the whole land. It's true to some situations right now. You can take the politicians as an example. When they are in the power and authority they have the tendency to abuse what has been given to them. They can use their titles to overpower others. Gandalf the grey, is yet another mage of the series has the opposite side with Saruman. He uses his power to stop Saruman's madness from domination. He can be compared to people that are observant and consciuos to the things happened in their society. They are the people who will make the first action if there are something wrong going on. They are the men of justice, for short. Power can blind one man's belief and actions. It may turn out as a tool of destruction and abuse. If you can use your own power in goodness, however, you will definitely turn the situations right and probably make a better result.

Being a fan of Tolkien's works, I gotta say I'm pretty obliged to answer your question here. Well, the history of Middle Earth stretched back to the First Age. That was very, very long before The War of The Ring. Okay, now the Istari, or Wizards, were sent by the Valar to Middle Earth to help Men and Elves against the growing shadow of the evil Valar, Morgoth. They were actually Maiar, and the term 'incarnate angels' is quite appropriate. They were sent to Middle Earth to inspire the inhabitants to do what they're supposed to do, not do the job for them. Thus, the main temptatation for them is trying to refrain from influencing others through magic to speed up the process. Their principle mission was to prevent Sauron, a fallen Maia himself, from doing such things. Okay, confused? Lol here's a no-bull approach to it. Think of the Valar as gods who created Middle Earth, the Maiar as angels, and Morgoth as Satan. There, easy. Morgoth was the first Dark Lord, Sauron was his servant. So when Morgoth was banished to the Void, Sauron became the Dark Lord. So apparently, the wizards are forbidden to use their power and influence over mortals. That explains why Gandalf, being a true wizard and carrying out his responsibilities dutifully, never wanted to wield more power or tried to exert his influence over other Men; he simply put his magic to good use, like an angel would. Saruman, the corrupted wizard, is basically a fallen angel, like Lucifer. Technically speaking, Gandalf and Saruman were on the same level as Sauron; they were just forbidden to reveal their true identities. Sauron gained much power and so even Gandalf can't easily defeat him. Hope my explanation satisfy everyone.

Because one of the main underlying messages was Power Corrupts, so if the wizards just went around using power randomly they would have become corrupt as Sauraman did. Gandalf used his power only when necessary and only for the benefit of those in his charge, never for his own gain. Those with power and wisdom knew how to hold back from using that power.