In a way, I guess you're right about joining later being a reason. But this doesn't apply to all Balkan countries. Only those that were part of the Eastern Block - under Soviet influence. And, I think we can extend it to some former Soviet Union countries as well. The reason these countries joined later is because they were literally isolated and suppressed for \~40 years (Yugoslavia was the only Eastern Block country who somehow joined). That has always been a thing: the West was allowed to develop, while the East couldn't. I brought this up because it's likely the reason why the West tends to look down on the East. I'm not saying everyone does that intentionally, but there is some sort of superiority complex there. The West sees itself as the pinnacle of progress and development (which, let's be honest, they kinda are, but only because history has been kinder to them) and feels threatened when the East is rising up. Even more so when considering who pumps the most money into the contest. That's why the Big 5 rule exists.

Speaking of money, that's also a reason why a lot of Eastern countries fly under the radar. It's no secret the budget is limited, so the staging might be more modest, thus not as eye-catchy. Televoting also costs money, so I can see people in the East being more reluctant to vote. Especially if they think it doesn't matter in the grand scheme of things, which tends to be the case.

There's also a matter of obscurity. Not many have heard much about the Balkan countries (except Greece and Turkyie), or some of the Eastern ones. It's related to the first paragraph. But yeah, basically it might be too unfamiliar to some, which might turn people away.

Ofc, foreign relations and politics play a role as well, which I'd say it's a weak point for the Balkans/Eastern Europe. Not that we don't vote for each other. At least, in Romania's case, afaik, we always give points to Moldova and vice-versa, if we can. But, as others pointed out, we're not as united.

Here's a little exercise: take a song from any Eastern country that you think it's a solid entry (from any year). If that same song came from the Big 5 or the Nordics, do you think there would be more hype around it? Would it have placed higher, or oven won the contest? We can do it in reverse as well: pick any song from a Western or Nordic country, that placed high in the finals, and imagine it coming from the former Eastern Block. Would the result be the same?