There is a dispute between Disney and Roku for the ad-supported version. It's also not available on Windows. Most likely due to revenue and ad sharing.
According to Disney Plus’s support website, the ad-supported tier is “not currently available on Roku devices.” It’s also not available on the Microsoft Windows desktop app, the site informs. So, at the moment, U.S. subscribers with Disney+ Basic or Disney Bundles like Disney Bundle Duo Basic (Disney+ Basic and Hulu’s ad plan) or Trio Basic (Disney+ Basic, Hulu’s ad plan and ESPN+) are unable to stream on Roku or Windows.
Disney told TechCrunch that it is still in talks with Roku about reaching an agreement that suits both parties. It’s our guess that the dispute is over an ad-share agreement as, by default, channels must enter an ad revenue split with Roku. Disney, however, declined to provide specifics. Roku also declined to comment on the negotiations.