Is your older version a Release, rather than a Beta? If so, that's why -- people using the Curse Client's default setting of only downloading Releases, or people downloading manually from Curse.com, will only see Release versions, not Beta or Alpha versions, so they'll keep downloading the last Release until you publish a new one.
If you want people to stop downloading older versions, you can either publish a new Release, or manually delete the older files from your project.
Betas are shown on Curse.com, but the big "Download" button (which is what 99% of users are clicking) still only gets you the newest Release version. You have to intentionally scroll further down the page and click on the Beta in the file list.
If all the files on your project are Betas, then the main Download button will grab the latest Beta, but once you release a Release, it will ignore Betas from then on (unless you delete all the Releases).