Because the Markdown parser “thinks” you want to write a, what they call it, “Setext” (structure-enhanced text) heading of level 1.
Like in
This is a Setext H1
=
Just add an extra newline.
Btw, this will also happen with one or more - for H2.
It was originally meant to more easily convert old text files to Markdown, where we used to write things like
This is a heading.
==================
Some text.
This is a subheading.
---------------------
More text.
Nowadays almost everyone uses the ATX heading style, since it offers heading levels 1…6:
# Heading Level 1
## Heading Level 2
### Heading Level 3
#### Heading Level 4
##### Heading Level 5
###### Heading Level 6
