Are CO2 cartridges not considered explosive?
I had a cartridge in my bag when going through airline security.


These are not uncommon. They are also used in for instance:


Life Vests
Ski Airbags
Soft air guns


In other words, it's not an unusual object, especially for holidays which I guess both your two mentioned destinations might be for many travellers. I've checked in my bag with an automatic self-inflating life vests a few times and while it went in the  hold never had any problems or found it removed. I believe there may be some limitations though on the number you can pass through, and they might start asking questions if you have more than two.


  I estimate roughly 15 airports (some duplicates) either didn't detect the cartridge or just ignored it.


It's not unjustified not checking them in. Onboard pressure changes, the cargo hold is cold and will shake during turbulence which just might trigger it to deploy if left screwed onto a life vest of similar.


  probably present more danger than other prohibited items, such as nail clippers


I can't see any real threat posed by these, except for the gun application, which of course requires a gun and not only a cartridge. Btw, TSA allows nail clippers.



Ryanair, the World’s favourite airline ;) states the following:


  Carriage of Self Inflating Lifejacket
  
  A self inflating lifejacket with 2 carbon dioxide cylinders (plus 2
  spares) can be included in either cabin or checked baggage.


In other words, they appear pretty liberal on the matter.