The amount of mercury intake with vaccine that was preserved using thiomersal is smaller than amount mercury taken into organism when you drink a cup of tap water in many countries, including tap water declared safe to drink.
While you may be right that the total amount of mercury is not an issue relative to other sources, allergies to thiomersal are a real thing. Here's a page on it: https://dermnetnz.org/topics/thiomersal-allergy/
Personally, I eventually discovered that my terrible problems with wearing contact lenses in the 1990's were caused by the thiomersal used as a preservative in the saline solution I was using. Given my known and tested reaction to having it in my eyes and on my skin, I would be very reluctant to have an injection that contained it.
Guess we should ban peanuts too, since allergies to that exist too. Some population being allergic isn't an argument to remove it, rather one for transparency and alternatives.
That's why it wasn't justified.