The do-release-upgrade tool checks https://changelogs.ubuntu.com/meta-release which is a text file that's pretty easy to read...
If you jump to the bottom, you'll see for 25.04
Dist: plucky
Name: Plucky Puffin
Version: 25.04
Date: Thu, 17 April 2025 00:25:04 UTC
Supported: 0
Description: This is the 25.04 release
Release-File: http://archive.ubuntu.com/ubuntu/dists/plucky-updates/Release
ReleaseNotes: http://archive.ubuntu.com/ubuntu/dists/plucky/main/dist-upgrader-all/current/ReleaseAnnouncement
ReleaseNotesHtml: http://archive.ubuntu.com/ubuntu/dists/plucky/main/dist-upgrader-all/current/ReleaseAnnouncement.html
UpgradeTool: http://archive.ubuntu.com/ubuntu/dists/plucky/main/dist-upgrader-all/current/plucky.tar.gz
UpgradeToolSignature: http://archive.ubuntu.com/ubuntu/dists/plucky/main/dist-upgrader-all/current/plucky.tar.gz.gpg
where the key detail is "Supported: 0", which means the release-upgrade tool ignores the upgrade as it's not [yet] supported.
For status on the upgrade path, please view
https://discourse.ubuntu.com/t/status-of-oracular-to-plucky-upgrades/59652
You can look at the blocker bugs, you'll note one (last I checked yesterday) is Fix Released meaning binary packages are available for download & thus can be used; others are only marked Fix committed which means that you can't utilize that code yet (unless you download the source code, compile your own binaries & alter other code to use that instead of what's currently available for download).
I suggest waiting, and watching the Status page if you're interested in the status of the upgrade path.
If you want to force it and take a risk, you could add the -d option, and https://changelogs.ubuntu.com/meta-release-development will be used instead, but if you value stability you're still best waiting.
The Ubuntu 25.04 released thus far is for NEW installs, ie. the ISOs were released.