My question is why we didn't take a simpler number, like $1~/~300,000,000$ ?
Well, the scientists were probably tempted, and one can only speculate as to the number of sleepless nights they spent agonizing over this very issue, but I think it ultimately boiled down to one simple reason, masterfully captured by <PERSON>'s insightful witticism :
A man with a watch always knows what time it is. A man with two watches is never sure.
Without denying that $3\cdot10^8$ is indeed a rather round number, please remember that the initial definition of the metre was the ten millionth part $($another round number$)$ of the Earth's quadrant, or half–meridian. Using the data available to us through the $1984$ World Geodetic System $(\rm WGS~84),$ on which the current Global Positioning System $(\rm GPS)$ is based, a more appropriate value for the speed of light would have been $$\tilde c~=~299~733~538~~^1!!{\Large/}!_2\quad\rm\Big[m\cdot s^{-1}\Big],$$ which guarantees the length of a terrestrial meridian to be as close as humanly possible to its historically intended value of $40~000$ kilometres. 840
$($Currently, it is closer to $40~008,$ and increasing c would only serve to make this difference bigger$).$
Both $($re$)$definitions, however, suffer from one fatal flaw : the rather unpleasant definition of a second as representing
the duration of $~9~192~631~770$ periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the $~^{133}\rm Cs~$ atom.
So why not just wipe the whole slate clean, and massively simplify things by redefining the metre as measuring
the distance traveled by light during $~30~^2!{\Large/}!_3$ hyperfine ground state transition periods of the $~^{133}\rm Cs~$ atom ?
So, all torn up inside between these three equally attractive choices, what were the poor scientists going to do, since, on one hand, all three are enticing, yet, at the same time, each also presents a certain flaw. So why even bother modifying the current definition in the first place ? Sure, it has its flaws, but, then again, so do all its rival alternatives. And, while they might each have their strengths, the current version also has its advantages, chief among them being its consistency, namely the fact that $($almost$)$ no previous measurement has to be changed or amended in any way, as has already been mentioned twice on this very thread. 840