Because I didn't bother to look it up. (And neither did you apparently)

You can read the exact guidelines here: https://apps.who.int/iris/bitstream/handle/10665/163636/WHO_...

In particular, they say:

Disease names may NOT include: [...]

- People's names [...]

- Cultural, population, industry or occupational references. Examples: Occupational, legionnaires, miners, butchers, cooks, nurses

What it again does not mention is brands.