How this humorless, depressed alkie/junkie ever got popular is the 8th wonder of the world.
To answer your question: Reid has been in the senate for 24 years. For many of those years Reid was a more moderate Democrat. He has definitely gone to the radical Left during the past decade, but he that was never the perception of him in the past. Meanwhile, the last time he ran for reelection he ran against a solid conservative candidate who had run a successful campaign in Nevada to define marriage as being between a man and a woman. Aside from only having a fraction of the campaign money compared to Reid, this candidate was considered by the RINOs in the state to be “too conservative” and “intolerant”. They supported Reid. Reid's campaign motto was, “Harry Reid: Independent, like Nevada”. He would tout his small town credentials and how he is such a good Mormon. A lot of people who didn't know better bought his lies. In the 1998 election Reid won reelection by only about 400 votes. The election was not contested, although I am sure that there were some illegal votes in the mix.
As the years have passed, Nevada population has gained more and more of three groups of voters who tend to vote Democrat: senior adults, Californians, and illegal aliens. Thus, Nevada, which was once a very conservative state, has become less conservative over the past few decades.