In what we can assume is a move to head off AMD's rumored RX 3080 branding for Navi, Nvidia has filed to trademark the numbers 3080, 4080, and 5080. Over the last couple of years, AMD has developed a penchant for mimicking Intel's branding nomenclature with its Ryzen processors and chipsets. Rumors have suggested for quite some time that AMD would employ a similar tactic with Nvidia, one-upping the RTX 20xx series with its own RX 30xx branding.
Veracity of the rumors aside, it would seem Nvidia isn't taking any chances, opting to preserve the branding for its Turing successors. Branding is important, and AMD has been anything but consistent with its own Radeon branding, at times choosing to be needlessly confusing. Nvidia isn't exactly innocent here either, pivoting from the recognizable GTX to RTX, begging confusion with AMD's RX series.
Both Navi and Turing cards using a similar branding would be confusing, especially to the uninformed buyer, and Nvidia certainly doesn't want its flagship cards eclipsed by Navi under the guise that higher numbers equate to better performance.
It's worth noting that while Nvidia has filed, the trademark is still under review by the European Union Intellectual Property Office. It's possible that the numbers could be too broad or common for a trademark.