Ubuntu Developers made the decision to continue shipping GTK3 and GNOME 3.38 in spring 21.04 release. It is noted that the transition to GTK 4 and GNOME 40 was considered premature due to concerns about the stability of a solution based on these branches. Given the significant changes in GTK 4 and the ongoing work to significantly modernize the GNOME Shell 40 interface, there may be problems with the reworked environment being fully ready for general use.
Preparing Ubuntu 21.04 is in the middle of a development cycle, and the maintainer team is overwhelmed and not ready to whip up significant changes. GNOME 40 is scheduled for release on March 24th, i.e. before the release of Ubuntu 21.04, there would be only about a month for the final identification and correction of possible errors, as well as correction of discrepancies with the applied add-ons and the theme.