With the advancement of cloud, ESB capabilities are not sufficient to manage integration between clouds, cloud to on-premise, and so on. Integration Platform as Service (iPaaS) is evolving as the next generation application integration platform, which further reduces the role of ESBs. In typical deployments, iPaaS invokes API gateways to access microservices. (link)
In general, microservices are fully cloud-native systems with smaller footprints. The lightweight characteristics of microservices enable the automation of deployments, scaling, and more. In contrast, enterprise ESBs are heavyweight in nature, and most of the commercial ESBs are not cloud friendly. The key features of an ESB are protocol mediation, transformation, orchestration, and application adaptors. In a typical microservices ecosystem, we may not need any of these features. (link)
Vikram: [ESBs are heavyweight. In microservices you do not need the features of ESB such as mediation, transformation, orchestraction and application adators.]