The critical difference between the ISO 20000 and ISO 27001 certification lies in their application. Their purpose and regulations clarify which industries or organizations are eligible for these certifications, respectively.
While the former has a narrower application, i.e., only IT-based services firms or organizations can get the certification, the latter has a much wider application. Almost every business sector and organization uses and stores information in any form, either manually or digitally, thus needing an ISMS. The ISO 27001 standard applies to all.