There are a few reasons why selling WordPress themes may no longer be a profitable business:
Competitors
One reason is that there is a lot of competition in the market, with many theme providers offering similar products at competitive prices.
Alternative Website Builders
Additionally, many website owners are now opting to use website builders, such as Wix or Squarespace, instead of WordPress, which reduces the demand for themes.
Free Themes
Additionally, the free themes available in the WordPress repository has also made it difficult for premium theme developers to sell their products.
Gutenberg Editor
The introduction of the Gutenberg editor in WordPress has had an impact on the market for WordPress themes, but it may not necessarily be the sole reason for the decline in theme sales. The Gutenberg editor has made it easier for users to create custom layouts and designs for their websites without needing a pre-designed theme. This has led to some users opting to use the editor’s built-in design features instead of purchasing a theme. However, there are still many users who prefer to use pre-designed themes for their websites, and many theme providers have updated their products to be compatible with the Gutenberg editor. So it might be one of the reasons but not the only reason for the decline in theme sales.