Well there is an explanation, albeit a politically incorrect one. All of the countries mentioned by Cornwell are ethnically pure. In the case of the Czech Republic, this entailed the murderous expulsion of more than three million Sudeten Germans and a later divorce from the Slovaks; as for Poland, its transformation from pre-war multi-ethnicity to post-war ethnic purity entailed tragedies of the highest order.
Croatia became Europe's latest state to achieve ethnic purity in 1995 when it murderously expelled the Krajina Serb nation. The international community's response was to treat the cleansing as a diplomatically convenient fait accompli.