Well because they did not always release multiple versions of a smartphone simultaneously, at least when you want to get it from carrier operator.
Let me clarify: in France you can get the phone "naked" without any carrier and pay the high price (in which case I guess that indeed both version will be available), OR you can get it from your carrier operator with a huge discount if you are a premium customer (I pay around 50€/month which guarante me to have the latest smartphone at 100/200€ once in a year).
However the carriers usually do not propose the "special" version of the smartphones when they're ,not sure it will sell properly.
Back in 2015 I could not get the Lumia 950 XL, had to go with the normal one...