It's still a suffering animal, so we should get the most data we can out of every specimen. That should minimize the total amount of mice being used.

Also only studying some mice can lead to biased results. Iirc in the pace trial participants who dropped out because the intervention worsened their condition where not included in the end result. With rats the researchers could just chose the healthiest ones and just claim they selected randomly.

Rant:

The pace trial was a horrible study on a chronic disease that was conducted by the british health system with the goal of denying care.
The trial also had other systematic flaws like having a laxer definition of healthy at the end of the trial that at the beginning, meaning people who where sick enough to participate would be declared cured at the end even if nothing changed.
This study is still used to denie people's lived experience and just call them lazy.
I don't have a personal connection to any of this, but learning about it made me angry enough, that I must share this knowledge.