2.4 GHz is one of the industrial, scientific and medical (ISM) radio bands. ISM bands are unlicensed, which makes it easier to certify the equipment with FCC (or its counterparts in other countries).
However, what special about 2.4 GHz? There is about a dozen ISM bands. Some at higher frequency, others have lower frequency. Not all ISM bands are international. But 2.4 GHz is an international band.
update:
Microwave ovens also operate at 2.4 GHz, which is not a coincidence.
Short version in Q&A format:
Q: Why does so much wireless communication operate at 2.4 GHz band?
A: Because it's an ISM band, and it's unlicensed, and it's international.
Q: Why is 2.4 GHz an unlicensed band?
A: FCC has originally set aside this band for microwave heaters (cookers, ovens). As a result, from the beginning, this band is polluted by the microwave ovens.
Q: Why 2.4 GHz for microwave ovens? Microwave ovens can work on pretty much any frequency between 1 and 20 GHz. There's nothing special (like resonance), when it comes to absorption of microwaves by water at 2.4 GHz (see also here).
A: The frequency choice was based on a combination of empirical
measurements of heat penetration for various foodstuffs, design
considerations for the size of the magnetron, and frequency
considerations for any resulting harmonic frequencies.
[These considerations were proposed by Raytheon and GE to FCC in 1946, when the decision about 2.4 GHz was made.]
The long versions can be found here. [This link goes to Indiegogo, because this bit of historical research was crowd-funded.]
Also, this FCC document (54MB) from 1947 can be of interest. Thanks, @Compro01 for finding this reference.
In my (short) experience, 434MHz (or even 915MHz) would be much better getting through obstacles than 2.4GHz.
Please mind that an obstaculized 100 meter range is quite large for the usual hobbist module. Also, antenna directionality is important: if the antenna is omnidirectional you might be radiating power to directions you don't need, so take that into account for your link budget.
Best Answer
In general, lower frequencies have lower free space path loss and therefor with the same power, same receiver sensitivity and same antenna gain will have longer range. However, as the frequency increases, antennas become more compact so if you are constructing a link with directional antennas higher frequencies can be advantageous.
https://en.wikipedia.org/wiki/Free-space_path_loss
http://www.ti.com/lit/an/swra046a/swra046a.pdf