That's because the system has been rigged by two parties to be that way. Third parties are also at a disadvantage because of the prevailing "they can't win" attitude of the electorate. I do plan on becoming involved with the Constitution Party after November. That won't keep me from voting for good Republican candidates, but it will keep my time and money away from a political party that is tacking ever leftward.
Most of us who are feeling disenfranchised by the Republican Party know the score, and will pull the lever for George W. Bush come November. At least, that's what I'll be doing. There's a lot to like about GWB - his "America first" foreign policy, national security, tax policy, some social issues...
Where we part ways is where he stands on domestic issues. Unilateral free-trade, CFR, illegal immigration, etc. He's not particularly strong on the 2nd amendment.
I do admire his honesty and integrity as well. Right or wrong, our President takes a stand on an issue and sticks with it.