> In another comment section (Reddit, I believe) there was anecdotal evidence that visitors from Bulgaria, Croatia, Cyprus, Poland and Romania regularly outstayed their visas and were notorious for violating a tourist visa, hence the additional checks the US wanted to put them through.

I guess it was anecdotal evidence, because the data shows the contrary:

- Bulgaria: 1.74% - Croatia: 1.08% - Cyprus: 1.35% - Poland: 1.49% - Romania: 2.06%

There are a few countries on the VWP that have higher overstaying ratios (Hungary, Chile, Slovakia). Croatia in particular has an overstaying ratio lower than Austria's and Netherlands'. Source: https://www.dhs.gov/sites/default/files/publications/FY%2015...

The reason they don't get VWP is the visa refusal rate, aka "US embassy in your country does not want to give you visa".