I don't know but I presume hollow blades are apparently hard to make and that there was a reason for trying to do it. The extra expense was somehow considered worthwhile.
It might only be 6 pounds but it's 6 pounds in something that spins very fast so there must be a lot of momentum stored. Presumably this makes the engine respond more slowly to throttle changes, for example. I don't know for sure because I'm far from an expert.
If the blades were hollow for the sake of cooling then that might mean that some other measure will have to be taken to prevent them from melting - with other undesirable trade-offs.