This is just my opinion, but I think it had a lot to do with lack of sales for wall art, especially photography in the art shows. I was in the art show business for 7 years and the first 5 years I had good sales, and was making my living. The last 2 years of the 7 years, sales had declined by 50%, and some show more then 50%. I thought maybe it was just me, but talking with other full time photographers, they were having the same issue with sales.
I noticed that at the time of my sales decline, there were a lot of big box stores offering inexpensive canvas gallery wraps. Home improvement Stores like, Ikea, At Home, Home Goods, Kirkland, were popping up everywhere. Even Walmart, and Meijer, which is a Midwest chain like Walmart that were selling cheap canvas gallery wraps.
Here is an image of an At Home store near my home, that has a wall at least 100 feet long, plus isles full of gallery wraps and some framed wall art.
The prices were far below what photographers were selling their artwork in the art shows. Below is a 36X24 gallery wrap selling at $39.99. I sold that same size gallery warp at my art shows for $199. I saw photographers selling 30X40 gallery wraps for as much as $500, and At Home 30X40 gallery wraps were selling on average at $79.
I believe as more and more people discovered these home improvement stores, and saw the cheap prices, they cut back on their purchases at the art shows.
Most of the photographers in the art shows that I knew that were making their living, like me, had all dropped out about the same time.
Lucky for me I had started teaching my Macro Boot Camps about the time my sales were starting to drop, so I dropped out of the art shows to pursue teaching workshops full time, and it has worked out well.
Most nature photographers that are making a living wage to support themselves are doing it through teaching. Very difficult to make a living wage from just selling your photos, as any of the full time pros I know are not able to do it.