Last year the European Parliament passed a resolution saying it "strongly regrets Serbia’s non-alignment with EU sanctions against Russia, which damages its EU accession process." There's also the problem regarding their relationship with Kosovo.
Also, only 34-43% of Serbians want to join according to polls. Compared to Albania's 97% and Montenegro's 79%.
I think Albania, Montenegro, North Macedonia and even Ukraine are more likely to join the EU next.