That's true, not a whole lot of Scots in comics besides a select few (Mirror Master II, Proteus, Moira MacTaggert and Wolfsbane). All these Eastern European villains and anti heroes are the result of comics that were written during the height of the Cold War. I mean Purple Man isn't even a communist villain the way other villains like Red Ghost and Crimson Dynamo are, as soon as he got his powers, he became a petty criminal and eventually devolved into a serial killer rapist. Purple Man's background as a Soviet spy was just an excuse to have him get exposed to experiential chemicals and get mind control powers, it really doesn't inform his character at all. You can easily have Killgrave be exposed to these chemicals through a different manner and it wouldn't even the character that much. Purple Man being from Croatia is not important as say Black Widow being from Russia.
So I would agree with you, there is no real purpose in having Tennant doing a Croatian accent, because Purple Man's Croatian back ground, never really informed his character in any real way. Loyalty to source material can be a good thing, but sometimes its better to do something slightly different for an adaption, making Purple Man be from the UK is likely just easier then trying to still have him be a spy from the former Yugoslavia. They made Abomination into a Brit, so I have no problem with doing the same Purple Man.