Same reason that LyX exists: LaTeX has a very steep learning curve, and is more intrusive than these alternatives.

To put it lightly, the syntax is odd. LaTeX doesn’t always pay off in the short term.

For actual writing, markdown gives you a way better flow.

What gives you the most fluent experience?:

    The house looked _very_ good. 

    The house looked \textit{very} good.

Because setting up an environment where you can compile LaTeX - including common packages/classes - can be difficult.