Randomly found an interesting post by Kevin Feasel on a blog called 36 Chambers:
The concept that most people who use the term “marketplace of ideas” want to explain is that they like to see the realm of ideas as one in which competition destroys the bad ideas, similar to how markets destroy inefficient production processes. But this misses the entire strand of competition dedicated to product differentiation, taking advantage of different preferences. As I said, people are more comfortable reading things with which they agree, and entrepreneurs in the market for ideas understand this, so they cater to certain subsets of the population. Thus, far from destroying the inefficient ideas, marketplaces can allow for niches. For example, Marxism was an ideology based on a theory which was shown to be patently false almost 140 years ago, and yet there are still Marxists today. [#]
In other words, some ideas are more like lawn gnomes than lawnmowers.
But I think something more substantial can be pointed out: with products in a market, the downside of purchasing products that appeal to your tastes but don't do what you want is much more prominent than with ideas. The downside with products is the spending of money, a tangible and limited resource. With ideas, the downside to adopting attractive but ineffective ideas is usually more along the lines of lost time, integrity, and opportunity, which are a lot harder to quantify and therefore not nearly as demotivating.