Because one detachment's cost is the other detachment's output, and no other pairing in the faction has that relationship.
Warpmeld Sacrifice is a self-harm engine. Each time a TZEENTCH MUTANT INFANTRY or MOUNTED unit is targeted, it can subtract 1 from Wound rolls against it for the phase, then suffer D3 mortal wounds at the end of that phase. Each time such a unit shoots or fights, it can add 1 to its own Wound rolls for the phase, then suffer D3 mortal wounds. A unit that does both in one battle round pays roughly four wounds on top of whatever the enemy actually did.
Sorcerous Invigoration is the only healing in the faction. Every time a Thousand Sons Psyker unit successfully manifests a Ritual, it heals D3 wounds. With Curse of Life (20 pts) that becomes 4 to 6 wounds.
There is one honest limitation to state, because it decides how you build the list. The healing applies to THOUSAND SONS PSYKER units, and it heals the manifesting unit specifically. Your Tzaangors are not Psykers, so the heal does not go to the mutants paying the sacrifice cost. What it does instead is keep your characters alive, and in this pairing your characters are the models carrying Bray Lord, leading the advance and generating your Rituals.
So the pairing is not a matter of sacrificing and healing the same unit. It is the mutants spend their bodies to win fights, and the healing keeps the leadership alive while they do it. That is a real synergy, and being clear about which models receive the heal is the difference between building this correctly and being disappointed by it.
The second reason is more direct: Relentless Rebirth, Ritual of Regeneration's 1CP stratagem, gives an INFANTRY or MOUNTED Thousand Sons Psyker unit Feel No Pain 4+ against mortal wounds, and it triggers on suffering a mortal wound in any phase. Warpmeld Pact generates mortal wounds on your own models constantly, and Channelling the Warp generates more. This is the pairing where that stratagem has the most triggers available to it.