Yes. On many Canon DSLRs, the expanded low setting (L / ISO 50) is not a true native sensor ISO. It’s typically achieved from the camera’s base ISO by extra processing, rather than giving the sensor more real highlight headroom.

In practice, ISO 50 is often similar to shooting at ISO 100 and then pulling the brightness down afterward. That can reduce dynamic range, especially in the highlights, compared with using the camera’s normal native ISO range. Because image quality is meaningfully compromised, manufacturers keep it disabled by default so users don’t select it without understanding the tradeoff.

The same basic idea applies to expanded high ISO settings too: they are usually processed extensions beyond the normal range rather than fully native sensitivities.

So the technical reason is that expanded ISOs are not part of the camera’s optimal, standard operating range. They require extra processing and may reduce image quality, which is why Canon makes you enable them explicitly.