Can somebody please explain to me side-opening doors on free-standing microwave ovens.

Why are they the most common?

It's an oven. Oven doors — the Aga notwithstanding — open downwards.

Downwards is better: latch doesn't catch your arm; door provides a surface for stirring, etc. And a side-opening door gets in the way if the work surface is on the other side of it.

And why do online shops not give this as an option in the search filters?

#FirstWorldProblems #microwaveOvens #oven #cooking