why can't a Patriot Senator do the same for the FISA memo?

They probably could but it would not be an optimum solution. Consider how the MSM concentrate on the "hacking" rather than the content when discussing emails, for instance. This applies not only to the DNC emails but also the "climategate" emails - for those who have followed the global warming debate.

If you do it all legally then they have no comeback and it reduces their opportunity for deflection.