Evens on main site vs microsite

I've got a client who serves as an industry resource and also puts on a large event every year. Historically the event was a subpage within their main site, but this year they launched its own microsite for it. Unfortunately the new microsite does not rank on SEO at all, the old subpage outranks it. I plan to advise him that he's better off keeping the event within his main site (not just for seo purposes but ease of management). Keep the new URL but point it to the main site page.

Do you wise ones agree with this strategy?

Also, is there any consideration whether it should be in a subfolder, or a subdomain, does it matter?

Please excuse my unfamiliarity with terminology, thanks!