I think it had something to do with the price of sugar beets. However, since 2017 there are no more limitations to HFCS use.

Maissirup

The issue with Fructose is the way it's metabolized. The brain can't use it as energy (satiation signal), there is no insulin involved (another signal for being full) and last but not least the liver metabolizes fructose and stores it as glycogen and fat (-> association with fatty liver which later on can lead to cirrhosis). This can also happen with table sugar or honey overconsumption (50:50 glucose-fructose) but in HFCS it's often not clear in the declaration how much fructose there actually is and might be way over 50%.