New DVD releases are often worse quality than their original versions. This is because companies can save money by using a single-layer disc instead of the dual-layer disc used in older releases. Single-layer discs have less space, which means they can’t encode as much video information.
For TV series, this usually isn’t an issue, but for movies and classic shows like Star Trek: The Next Generation, it can be noticeable. The original DVD master is re-encoded to fit into less space, leading to a lower-quality image with reduced detail, especially in motion.
Companies are making these lower-quality releases because physical media sales have declined, and they believe the difference won’t be noticed by consumers. However, this practice can be frustrating for fans who want to own and preserve the best version of their favorite shows.
If you’ve already bought a DVD set, check its size to see if it’s a single-layer disc. Look for Blu-ray versions instead, but be aware that some upscaling may introduce visual artefacts. Buying used discs from earlier production runs can also help avoid lower-quality releases.
Source: https://www.howtogeek.com/im-furious-about-this-modern-dvd-practiceand-you-should-be-too