When was the last time a Representative was elected to the presidency?
When was the last time a candidate suggested that we nuke an entire population elected?
I think the division between people who live in a fantasyland where Tancredo can win and those who know he can't is similar to the division in the Democratic Party between people who think that, say, Howard Dean could have won a general election and those who knew he cannot.
It's a question of how much of a conservative "Bubble" one lives in. People who have entire families of Republicans who live in extremely Republican areas and go to churches full of Republicans and really don't have regular contact with actual Democrats at all can get some very weird ideas of who is electable and who isn't, and also can get the idea that their own personal hobbyhorse issues are just as important to the rest of the country as they are to themselves.
Like I said it's not purely a Republican problem, the progressive core of the Democratic Party has it worse.
But anyone who thinks Tancredo is remotely electable is absolutely, positively nuts.
"Tom Tancredo cannot win. Not in a gazillion years.
Why?"
Because the people who actually run the Republican party don't want him to?
To answer your question "Why?":
Unfortunately, you do need to garner votes from non-Republicans and a few democrats if you're to win a national election in this country. It's why all politicians, of all stripes, "move to the center" during a general election. There simply are not enough R's and D's to survive on one vote alone.
There are "Independants" (i.e. indecisive folks waiting to hear which side will bribe them best). There are "Libertarians" (i.e. Republicans with substance abuse issues masquerading as principles). You do have to make an effort to appeal to these folks if you wish to get their votes.
Tom Tancredo appears to many of these people as another in a long line of grim-faced, conservative stromtroopers who kick puppies and take away children's candy because he is vocal on the illegal immigration issue. No appeal to the squishy center when you want to ship George Lopez or Carlos Mencia back home.
The onliest positive name recognition he has is with the right side of the right. With everyone leftward of that he is tarred by the whackjob image of the even more 'extreme' right.
not only would Tancredo lose but he would do to the Hispanic vote what Goldwater did to the black vote