I don't think very many 2nd Amendment cases have gotten to the SC, so many judges never get around to addressing the issue or even thinking about it very much.
Actually *because* so few second amendment cases (the last being in 1937) are ruled on by the Supreme Court, lower court judges actually have more opportunity to rule on them, since the field hasn't been occupied by the SC. The ninth circuit certainly has addressed the issue, as has the fifth and several others. The DC court of appeals is less likely to have done so, especially in the short time Roberts has been serving there.