At the time, with the Vietnam War really beginning to build up in 1966 with men, material, and spending...and with the US Navy not wanting the type of downdraft a four engine vertical landing would necessitate...they simply decided not to go with a VSTOL aircraft at the time.Do you know why it was ultimately cancelled? If it was due to budget concerns, then it would be quite unfornuate...
I personally believe they missed out on an opportunity (and in retrospect I believe that feeling is borne out)...but at the time that type of opportunity was just not very high on the priority list.