For one, the company that sells ivermectin in the US (Bayer) does not currently have an alternative covid treatment. They have publicly stayed that they do not believe ivermectin helps against covid. Secondly, an RX of ivermectin (for humans) appears to cost more ($35) than the vaccine ($20). Additionally, the vaccine is already paid for, whether or not people use it, many countries (especially the US) pre-purchased hundreds of millions of vaccine doses.
Given that vaccines are already sold and adding ivermectin is pretty safe and is being added to the cocktail of treatments, already being used, I see no financial incentives for drug companies to discourage it.