That could be a good topic for someone's masters thesis. It is a mix of many aspects, one major one could be joining late, as already mentioned. By the time we joined, some countries like Estonia had already about 15 years of experience, and the foundation of this relationship was built in the 90s when the contest was much calmer than the chaotic late 00s.
Sending the most popular band on our fist attempt, finishing dead last and after that, the band giving interviews to all media saying ESC is a bizarre circus - opinion which I think stuck with many people. Then we withdrew for many years and although the contest changed a lot for the better once we returned, I think people just didn't bother to give it a chance.
And, probably the main reason - Czech journalists. This piece was published on the most read Czech newspaper after this year's final.