The push to create a closer-knit European banking system is struggling to overcome a basic hurdle: Lenders aren’t merging.

Within Europe there were only 15 deals to merge or acquire stakes in other banks this year, according to Dealogic, the lowest tally in five years. Uncertainty over incoming banking rules, regulators' wariness of agreeing mergers between weak lenders and doubts over the political future of the eurozone are weighing on deals, bankers say.