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Perhaps due to the sun's rotation?

I'd guess that it's related to the sun's rotation - the sun's equator roughly lies on the ecliptic.

https://en.wikipedia.org/wiki/Solar_rotation

I believe it is the fact that the solar system was once a spinning ball of gas and dust, and the spinning motion caused the cloud to flatten out into the protoplanetary disk. Like flattening a ball of pizza dough by spinning it in the air.

https://en.m.wikipedia.org/wiki/Protoplanetary_disk

https://www.vanderbilt.edu/AnS/physics/astrocourses/ast201/o...

When the solar system formed, the gas cloud collapsed along the shortest axis first into a spinning disc and then formed the planets and sun. This naturally aligns the largest bodies in most systems along the astral axis.