At this point, keeping her bounty low is an excellent move by the World Government.
Remember, the public doesn’t know anything about Nico Robin. The reason she got her first bounty of 79,000,000 was because she escaped Ohara, yes, but the World Government reported it using the fact that “she sunk 7 battleships” as justifications for her bounty.
For an 8 year old girl on the run, this raises eyebrows and let the masses know that she’s dangerous. Then she disappeared for 20 years and resurfaced in Alabasta after Smoker confirmed her whereabouts. Aokiji verified this info and the events of Enies Lobby unfolded.
Now, the World Government always knows Robin’s whereabouts – she’s guaranteed to be wherever the Straw Hat Pirates are. Increasing her bounty so that it stands out among the rest of the crew paints a target on her back, like it did Zoro back in Sabaody. The Government doesn’t want people to know that Robin can awaken the Ancient Weapons, or that she can read the Poneglyphs.
So keeping her bounty low hides the fact that she’s secretly super important, and prevents her from being singled out by common bounty hunters.
Why go for Robin when Usopp, Sanji, Zoro, Jinbe and Luffy have much higher bounties? It’s better to try and secretly capture Robin, which is the Government’s preferred way of dealing with her, than having some random capture her and force her to spill the beans. Or, the Government can focus their attention on capturing the Straw Hats in general, thus capturing Robin that way.
*by Waternomnom/Therrester
