X is an abbreviation for Christ. I don't think that's the same reason as Pax.

I can't provide a reason for pax, other than it's easier/clearer than pas, which looks like a typo. But it's entrenched in the lexicon.

Y = IATA code for Canada, plus "Vancouver".

It's much more logical than other Canadian codes, like YYZ (which seems to have been named after a Rush instrumental;))

That's because when they moved from two letter to three letter airport codes, some airports just added an X where there wasn't a logical third letter.

If Los Angeles' 2 letter code had been LO, the new code might have been LOS, but since it was LA they chose to add an X. Same thing happened to Portland, which was using first and last letter (PD to PDX) and to Phoenix (PH to PHX). For Phoenix it actually works out okay because of the name.

The OED traces it back to 1978, with a usage in Aviation Week and Space Technology describing a plane as holding xxx pax.

I think the use would predate this?

A bit more digging reveals that PAX is the SITA TELEX abbreviation for Passenger, which is likely how it found its way into aviation jargon. The reason why they chose PAX rather than PAS I don't yet know. I did find out that PAX is both the singular and plural form, and I'm speculating that this might be why. PAS just sounds a bit more singular, as in 1PAS and 2PAS doesn't sound as plural as 2PAX, but I'm stretching.

Probably totally unrelated, but PAS has a different meaning in the list of U.S. Air Force acronyms:

PAS - Political Affairs Strategist
PAX - Air passengers

This list that I found on Wikipedia is a total hoot, BTW, and contains terms like PICNIC, which stands for "Problem in chair, not in computer."

The term PAX is a very old term originally used by telegraph posts to denote passengers. My first job, on the railway, still used PAX between signal box telegraph communications and dates back, I believe, to the invention of morse based communication. I believe the 'X' was applied in place of a double S, as the single letter was rarely used, is a reasonable illiterasation of SS, and is, of course - cheaper.

For the same reason you drive on a parkway and park on a driveway?

It is all down to the the early days of the telegraph where messages were paid by the letter. To avoid unnecessary costs words were often abbreviated, X was chosen to end the abbreviation as it was an uncommon letter in normal use. This probably started informally amongst the telegraph operators who were incentivised as they were paid per message sent. It was eventually officially adopted by the telegraph companies and subsequently by radio telegraph operators. Radio and telegraph operations were then coordinated wordwide by the the formation of the International Telecommunications Union (ITU) who still regulate radio operations today.
One of the main things they coordinated in the early days of radio was the use of radio on international transportation introducing amongst other things a system of callsigns for radio stations. Each country who was a member of the ITU was allocated a call sign prefix, either a single letter, two letters or a letter and a number. Eventually these callsign prefixes also were used as the initial letters for civil aircraft registrations.

LAX was already taken.

One should also remember that there was an idea of US 'International Airports', once they recognised the rest of the world, such as IAD (International Airport Dulles), IAH (International Airport Houston) and possibly a couple of others. But that idea seemed to fade as fast as it started ... IAN never caught on in NY :D

Prior to the establishment of three letter airport codes, several airports in the US were referred to by there two letter National Weather Service Code (e.g. LA for Los Angeles, PD for Portland, OR, PH for Phoenix) while other airports were spelled out. IATA simply appended X to the NWS codes where they were widely in use.

Although most airports in Canada have IATA codes which start with Y, not all do; there are several which start with X (e.g. XBR, Brockville Regional Airport) and Z (e.g. ZBR, Bathurst). Additionally, not all IATA codes which start with Y are in Canada - e.g. YNG (Youngstown), YIP (Ypsilanti)

I think this is as close as we're likely to come. The need to minimize keystrokes led to abbreviations, and X is an uncommon letter unlikely to be confused with anything other than an abbreviation.

Why is is PAX not PAS?
 

PAX isn't a three letter city code anywhere in the world. PAS, Paris National Airport, Greece, is.

Why is is PAX not PAS?
 

Sorry, Paros National Airport, not Paris.

PAX = Port-de-Paix, Haiti, and we've already documented that the use of PAX goes back to the days of the telegraph, before there were airport codes (at least to any extent.)

Of all the abbreviations to complain about, why this one?
Just be at peace with it.