why switch negative

Thick skinned.....not think.If it doesn't fit, you need a bigger hammer.

Most outputs on an alarm system are negative output.  That is probably the reason that most diagrams here are as they are.  It does not matter which one gets switched.  It all works the same.Let's Go Brandon Brown. Congratulations on your first Xfinity Series Win. LGBFJB

Also, a convenience factor, if you're supplying the relay from a switch, it's often easier if not illuminated to take the other side of the switch to a ground.

Also, if the control wire is ground switched, if it inadvertently gets shorted, it'll just turn on the relay, as opposed to blowing a fuse or melting down wires. Alarm modules switch negative also because they can drive more current in those types of transistors, and it doesn't require any additional power from the alarm power input. If you have multiple outputs on at once it'll add up. Switching negative allows the current to be drawn through the ground wire.