Good point, we aren't generally allowed to own RPGs and attack helicopters, so it's not maximalist. However, RPGs and helicopters were not around at the time of the passing of the amendment, so I'm not sure we can that it is historically and textually accurate. For that matter, the guns this legislation would target weren't around then either, so I'm not sure how useful that criteria really is.
Because there was essentially nothing in between 1789 and the rise of the modern "gun rights" movement starting in the 80s to support it. The current interpretation would have been completely preposterous in the 60s and 70s. Just see how gun laws were used against the Black Panthers in the 60s.