Why there is no airport IATA code XXX? It seems that there is no XXX IATA code: from Wikipedia, XVL (Vinh Long, Vietnam) is followed by XYA (Yandina, Solomon Islands). There is a source claiming that XXX used to refer to Mena Intermountain Municipal Airport, but from Wikipedia this place seems (rather strangely) to have no IATA code at all (and a source for this claim is given).

What are the reasons for not having the XXX IATA code? Is this a reserved combination? Has it belonged to the Mena Airport in the past, and if so, why has it been deregistered on October 21, 2014, as the source claims?
It looks to me like the source site you quoted is unreliable or at least not current, e.g. it gives results for codes QQQ and ZZZ too, but they haven't been assigned by IATA. The actual IATA code for Mena (ICAO: KMEZ) seems to be MZX.

Anyway, there's no particular reason why XXX, ZZZ or any other specific combination should be assigned; they just haven't been used yet. There's also no reason why IATA should assign a code to every airport: the US alone has over 5,000 public airports but only 500 or so have air carrier service.