Ross answers Frum’s critique:

First and foremost, because it seemed to us that there was value, given the American Right’s present straits, in writing a book that focused very narrowly on domestic policy and debates over the welfare state, to the exclusion not only of foreign policy but also the domestic controversies over abortion, church-state separation, gay marriage, pornography and sundry other issues that have preoccupied conservatives for many years.

It would have been a pretty glaring error to exclude discussion of foreign policy if “Sam’s Club Republicanism” and Ross and Reihan’s reform conservatism generally necessarily entailed some significant shift in foreign policy, but from what I understand of them it doesn’t.  Not only is it true, as Ross says, that wading into the details of welfare policy is already controversial and involved enough without tying it to arguments about other kinds of policy, but because of the nature of the project my impression is that there is simply too much material to cover on this specific area to give any treatment to other, equally weighty matters in a manner that would give them their due.  No treatment can sometimes be better than cursory treatment.  Finally, there is a question of thematic coherence (to say nothing of Ross’ point that the co-authors may not see eye to eye on everything).  If you’re writing a book about a specific kind of social policy, you’re not going to spend much time thinking about NATO expansion one way or the other and there is no good way to tie in these other issues, short of dubious “national greatness” handwaving that somehow meaningfully links security policy with entitlement reform.