To over-simplify it to a shameful degree, because Ukraine's a little fucking complicated, Ukraine is the passe between Eastern Europe and Europe proper. Ukraine may be a bit of a mess, but when your options are Belarus, Estonia, Latvia, Lithuania, or Ukraine, it's not much of a choice. You can take a risk and try to work across the Black Sea and strike a plan with Bulgaria, but Bulgaria frequently forgets it's not 1980 anymore, so they're lagging a bit behind and outside of Sofia it is not a very well-developed country, so it would be difficult to hide large-scale "businesses."
That being said, if you for whatever reason ever decide to visit Bulgaria, don't go to Sofia. It's the Los Angeles of Bulgaria.
A lot of the Russian oligarchs and organized criminals move their business through intermediaries in Ukraine so they can make their way to and from Europe and the rest of the world. It's not that Ukraine itself is particularly lucrative or important on its own, it's that it's a geographical ideal with a nicely-sized economy and development, that just happens to also be full of impoverished people and notoriously corrupted politicians, both of whom are incredibly easy to bribe.
It's also the country with the largest GDP, a powerful energy sector, industrial base, the most development, and it has the most alliances to well-developed nations of all the countries on that list, so it's not going to be as suspicious if large amounts of money or large deals change hands in Ukraine, where-as the same amount of money or trade deals passing through Latvia are going to attract a lot of attention. If Hunter Biden took up a position in Estonia where he was paid $85,000 a month, he'd draw quite a bit of fucking attention in a country with a GDP of around $20-something billion where your average person is a goat-herder living in Soviet-era apartment blocs, instead of a country with a GDP of $130b+ that's more than developed enough to house plenty of legitimate businesses that do not involve herding goats.
That's not really a fantastic explanation, but off-the-cuff that's about the best I can try to explain it.