Having their endorsements staying focused strictly on gun rights accomplishes two goals. First, it keeps things focused on the primary mission. It can be very destructive to an organization to let itself get diverted from the primary mission by getting caught up and distracted by arguments over areas that the NRA has no particular expertise and no reason to waste resources developing it, not to mention it can cause an organization to splinter into various factions instead of remaining a united front. For example, the NRA has no business getting into the middle of the abortion issue, to choose on particularly divisive issue. If I want advice on a politician's position, I'll go look up the Right To Life endorsements.
Second, staying focused on gun rights increases the NRA's effectiveness when lobbying. Face it, the NRA is the most effective lobbying entity when it comes to defending the right of its members and all Americans to keep & bear arms as guaranteed by the Second Amendment. The NRA needs votes from both sides of the aisle to advance that agenda. For example, here is what David Koppel had to say about Harry Reid.
Harry Reid’s victory in Nevada means that pro-gun bills have a chance of being brought to floor, or offered as amendments to other bills. Had Reid been defeated, the Majority Leader would have been Charles Schumer or Richard Durbin, both of whom are highly-motivated and well-informed on the anti-gun side.
It wasn't until the Kagan vote that they unendorsed him because of all the flack they got from the Tea Party and threats of quitting from members.