There might be reasons you might not rush with update to next major version. Imagine you run a big Bitcoin business, with a lots of infrastructure and different integrated software, you need to carefully test everything before upgrading. That may take a time. Especially if you run into problems which needs to be fixed. Bitcoin Core breaks some RPC API backwards compatibility with default configs from time to time, for example.

So, it's standard software industry practice to keep maintaining older major versions for some time.