It's a niche market. Very few people out there are in the market for fish finders compared to those in the market for large flat screen TVs. There's also very few manufacturers, and when one comes out with a new tech people want and will pay for they can pretty much charge what they want for it.
You can say the same thing about computers and tablets and cell phones. Why pay $1200 for a new phone when you can have a laptop computer for the same price with a much bigger screen and a keyboard for a few hundred bucks less?