Honest answer, fear of Russia. Baltic states and Poland know first hand that given a chance Russia would gobble them up so they focused on getting into NATO and EU while Russia was still weak.

Moldova, Ukraine and Belarus stuck with CIS (Russian sphere of influence) and it hasn’t paid off for them. Ukraine got what Baltic states and Poland feared (which is why they’re one of the biggest supporters of Ukraine) and Moldova went “oh fuck, Russia can’t be trusted” as they decided to go down the path of leaving the CIS and joining EU and NATO.

The countries who decided to embrace the west got a better deal than the ones who decided to stick with Russia.