Spectrum management. Your option 1.
It’s also a pretty good middle ground for signal propagation and energy use and speed and that sort of thing as far as I know.
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Because of law. The law is that you can't just transmit radio. The governments of the world have enclosed the electromagnetic commons. Transmitting radio is illegal except as the law allows it. 2.4Ghz is a carve out in the law for unlicensed use. In the USA we call that portion of the law "Part 15"
https://www.ecfr.gov/current/title-47/chapter-I/subchapter-A/part-15
Since radio spectrum is shared globally because radio waves don't follow political boundaries, it's coordinated by a UN body called the ITU, the International Telecommunications Union.
There are a few frequency ranges set aside for so-called ISM (Industrial Science and Medical) use.
The 2.4 GHz band was set aside for such use and together with Wifi, Bluetooth and baby monitors, it's used for all manner of other things.
Since the range at low power is quite short, many devices can share the same frequency and not interfere with each other.
Here's some background.
Bluetooth because it's named after Harald Blåtand and it was two point from end to end so the first and last letters B and D are 2nd and 4th in the alphabet so 2.4 and WiFi because it is Bi Directional so that is 2 and D=4 as above, also 2.4. Funny coincidence, really.
Also the width of the pyramids and the distance to the far side of the moon.
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