The simple descriptive (rather than ideology-related) answer is just that it's really unpopular with a majority of Republican voters. So even if you're right th

The simple descriptive (rather than ideology-related) answer is just that it's really unpopular with a majority of Republican voters. So even if you're right that GOP ideology ought to consistently favor it, and even if some party leaders would actually support it, a pro-amnesty Republican probably can't win a primary.