He is coming off a terrible season in a contract year, was making $13M+/year and is represented by Scott Boras.
Nick Johnson's production has been steady, and he's coming off a $5.5M/yr contract. He can be had for less and will get on base more than Beltre, which is more of what the Giants need.
Heathramos is probably right in thinking that the savings of signing Johnson over Beltre would help toward another signing.