As a matter of interest.
I presume that using 121.50 for CAT contact is seen as an adnormal procedure.
And as such will be logged as an "emergency" I also presume that such activitys by D&D are logged. How many times a year is this function used in the UK?
How many ATC units have access to transmit on 121.5?
How many of them can recieve it?
Personally I think that its a perception thing. One side who presume its a mission critical service. And another side, who the SRG agree with, who reckon the safety case of GA being D&D scared is a greater risk to safety than all CAT not having box 2 selected to 121.5.
I presume its all down to risk assement as usual. If the numbers of CAT which were required to be contacted by 121.5 to restablish contact are high enough to cause it to be a primary service it would have priority. I suspect though it is used very infrequently if at all in the UK, the number crunchers and have done there magic and reasoned that this multi million pound service is better served by being used by GA by giving fixes to, to avoid controlled airspace.
I would also presume that if such a contact freq is required it would be far cheaper to create a seperate "missed coms" freq for CAT which doesn't require the VDF infrastructure or training. Every AtC unit in the land could access it and transmit only. You could proberly pipe it through 4 telly masts through the UK or gawd forbid the classic FM masts. You can pick that up everywhere even if you don't want it.