There are too many other options available in midfield
Chelsea had one of the deepest squads in Europe before the unfortunate January transfer window as outgoing signings, injuries, and suspensions have hurt that depth. Even with all of these factors, there are still many options in midfield and on the wings that could provide more productivity than Sancho at the moment.
This problem will become even more exacerbated next season with the hopeful return of Mykhailo Mudryk from suspension and the arrival of players like Kendry Paez and Estevao Willian. Obviously not all of these players are traditional wingers but they would take up one of the midfield spots and will ostensibly be cheaper than Sancho as well.
Sancho started his career as one of the most promising youngsters in Europe and was lighting up the Bundesliga for a couple of years before returning to England. His career progression has stalled unfortunately, and Chelsea would do well to explore other options rather than committing to him long term.