Impressive himself doesn't qualify for Foundation Registering because he is only 24% foundation.

When most of the Foundation Associations were starting up, it was right at the height of the HYPP scare. Many people still didn't know much about the disease and how to control it. My guess is that the Foundation people simply decided it would be easier to exclude Impressive horses from their registery than try and deal with the mess. They still hadn't come up with the test yet and there is no N/N, N/H or H/H status back then.

Where HERDA came into the picture, these associations were up and running with many members linebreeding Poco Bueno. Again, my guess would be that it was easier to do nothing when it came to HERDA than to alienate a large percentage of Foundation members.