This is constantly said on these forums. I don't know why. I know O'Reilly and Fox constantly say Cruz is "too conservative" to win in the general election, repeating the same arguments they made for McCain and Romney that went down in flames.
The polls show Cruz can beat Hillary and does better against her than Trump.
http://www.realclearpolitics.com/epolls/2016/president/2016_presidential_race.html
If the polls are right, then I hope I'm wrong. I just don't personally see how he can beat Hillary. If she has to drop out, the game changes. But that's a big if.