Simple: if you make your documents in Markdown, you won't be making them in Google Docs. Stuff made in Google Docs is sticky, increases your dependence on Google, and importantly has a valuable network effect due to their collaboration features. It is in Google's best interest to encourage you to rely on their proprietary services instead of offering you alternatives that allow you to switch provider easily.

Additionally, the "people who know what Markdown is but also like trusting Google with their files" demographic is vanishingly small and not who Drive is targeting at all.