Libertarians believe that laws should be
1) Minimally invasive in peoples lives
2) Only created to address issues that cannot be dealt with more effectively by private parties acting in their own interest.
3) Applied equally to everyone.
The objective is to achieve the maximum personal liberty possible while not sacrificing the good of the community (maximizing personal liberty regardless of the community is called anarchy btw). Requiring everyone to purchase specific goods or services does not meet this definition because it is not the optimal solution for everyone, places unnecessary artificial constraints on personal liberty, destroys competition (which harms society as a whole) .
Health insurance can and should be handled by private parties based on their own interests. Inserting the government into such a market in the first place is therefore a violation of everything a libertarian believes in.