There are some key reasons for this.
We do not typically migrate questions to beta sites. While it is not specifically stated on Software Recommendations itself, Area51 does still list the site as in "public beta" as of 2015-10-06. Beta sites are preferred to grow organically to prove that they can survive without content being pushed to them.
According to this post on SR they are currently in the process of graduating. Until such time as they are fully graduated I would consider the site to not be a "full" migration target.
Quality. In all cases migrations are subject to the golden rule. A lot of the questions we get are "plz gimme softwre to make blah" which should be closed rather than migrated, opening up the migration path just means that those question are quite likely to be moved rather than outright closed.
We only have 5 migration slots, who would we remove to make space for Software Recommendations?
So for me we need at a minimum for Software Recs to be out of beta, but after that we need community consensus on which site we use least for current migrations.
Given that SR is graduating though, it would be good to see stats on the current migration targets and whether SR could be a better target.