something tells me that the traitors used up a significant portion of their assets in the siege. even if they took 25% losses(actual destroyed suits) how many of the remaining were either badly damaged or had to be salvaged to make up other suits missing components? so let's take that number up to 50% total remaining suits.
now over 10k yrs how much attrition has occurred? so let's conservatively bring that number up to 60-70%, so out of the 1000(just using round #) each legion started the heresy with, they would have 2-300ish remaining thru to the Scouring.
Now factor in the shattering of legions, warbands, petty squabbling, lost in the warp and the recklessness of chaos would lend itself to burning thru the best materiel & equipment. Wanting to maintain your equipment is not the same as having the resources to do so.
Now on the loyalist side, they have viewed the TDA as Sacred and only entrusted to those worthy. then only used in the most appropriate engagements and retrieved if lost. sometimes even expending significant resources to regain said suits. Now if you say that only 500(going from same #s as above) survived for loyalists, the rate of attrition would be lower & rate of repair would be higher.
so overall, I would say that most of the Tartaros & Cataphractii suits the traitors had would've been destroyed/rendered unusable by scouring(earliest) & M32-33. Any remaining are either, mutated/become one with wearer or in a state of disrepair that they've been ditched for Indomitus pattern(more readily available due to new traitors joining from equipped chapters).
Irrespective of my feelings for traitors, they should have at least one or two units of Saturnine armour just cuz....