Only about ten percent of the population is left handed. You then have to ask what percentage of those people are shooters. Probably about 2% or less of the actual shooting population. You would be targeting a very small market. That would not be a very good business model.
Niche markets are often pretty good businesses, especially if you are the only one catering to that market.
Consider that you have 200,000,000 gun owners, and 10% of them are left handed that might want a left handed gun, 2% of them would definitely want a left handed gun. At 2%, you have a minimum customer base of 4,000,000 people that want a left handed gun. If they were more popular you might be able to convince that other 8% that they want one, and then you have potentially 16,000,000 more customers. At 800 dollars a firearm for 20,000,000 people you are talking about a "small market" potentially worth 16 billion dollars.
They probably wouldn't make 16 billion dollars. But if 4 million people wanted a left handed gun, it is still a multi-billion dollar industry. The hurdles are developing the tooling for making a quality left handed option where these guns can be produced in a cost effective way. Few people want to pay $5000 for their left handed gun, but if they could be sold in the 800-1000 dollar range I know I would definitely consider owning one.