The developer has come out and said that as of the announcement of them dropping support for Mac/Linux, the two platforms combine make up 0.6% of their active player base [1]. I am guessing they just did the math and decided the cost of switching APIs to something like Vulkan is not worth it when such a small portion of their users would benefit from it.

[1]: https://arstechnica.com/gaming/2020/01/rocket-league-will-dr...