Many modern systems are built around data. So, data engineering and systems engineering are very close.

It's mostly personal bias and circumstances.

Data engineering and systems design are incredibly close. After some time, they completely mesh together.

And from personal experience, the knowledge-volume to become practically potent is actually smaller from data engineering side (than from normal software development side).

I've found it easier to get data gigs and simultaneously strengthen the knowledge.

(I love databases, so that's also a factor)

The bonus $$$$ is also a nice side effect.