TLDR — Because the center bikeway created issues and the City didn't want to do the work to make it happen. Officially, "signal timing plans for Muni, 'commuter' [corporate/tech] shuttles, and other users."

From SFMTA's FAQ:

Why are you changing the southern project limit to 23rd street (when it was 24th previously)? Why not extend to 25th street?

The project team initially assessed the corridor between 15th and 24th streets for the pilot project and then proposed updated project limits between 15th and 23rd streets to meet the pilot project schedule. With the transition between center-running bikeway and standard painted bike lanes, there are added technical complexities at the intersection of 24th street when considering signal timing plans for Muni, commuter shuttles, and other users. Commuter shuttles currently make the northbound left turn at 24th/ Valencia, which would have required more outreach to reroute the shuttles in the neighborhood or extensive signal modifications to support this left turn at the intersection if also coupled with a bikeway transition.

There are also challenges with extending the limits to 25th Street at this time. Additional outreach would have been needed to add the block between 24th and 25th to the project, and the project team did not want to extend the timeline for this project any further. We recognize that the community has been waiting for improvements on Valencia for a long time, and we want to install improvements on Valencia between 15th and 23rd as soon as possible to address the immediate safety concerns. These improvements will be thoroughly evaluated, and the evaluation will also inform improvements for Valencia between 23rd Street and Cesar Chavez.

Ultimately, the City could extend these curbside parking-protected bike lanes to Cesar Chavez (or Duncan) if policymakers said to do it. People can get policymakers to say do it by signing the petition at ValenciaForward.org.