I can imagine, one reason is due to hygiene. The lotion is in bottles and closed, but a toothbrush, you will use in your mouth, a more sensitive part, where some people cares about things like hygiene more, that using a lotion, although it is wrapped in a box ore anything like this.

I know several people, who do not use the toothbrushes in planes because of hygiene reasons, they bring their own.

And the most hotels offer them on request, also the low budget hotel like Ibis, Etap or Express by Holiday-Inn.