Why can't Bitcoin switch over to Gridcoin's proof of work system?
Such a switch will never happen. There are three main reasons for this.


It would require the worst kind of hard fork, one where a lot of legacy code would have to remain in the coin, for the sake of validating transactions before the hard fork date.
While the reward system itself has been specifically designed to not be game-able (https://github.com/TheCharlatan/Gridcoin-Research/tree/master/contrib/BoincMonteCarlo), it is nevertheless destroyable by a malicious actor with reasonable funding, since it relies on a centralized service to distribute its data. 
There is little to no research towards the specific crypto algorithms used to secure user information in the chain, such as https://github.com/TheCharlatan/Gridcoin-Research/blob/CPID-style/src/cpid.h . In fact the CPID (which is used to identify researchers) is an md-5 hash of some string and the user email, so it definitely does not compare to Bitcoin's standard of security.