The game knows what game modes you need and doesn’t give them to you. Needed one ctf game, got all my challenges done and didn’t get one. I swap it and the next game is ctf. The game was built to sell swaps.
i’ve said this for weeks and have posted about it a few times even. typically people just say its the “gamblers fallacy” or whatever. but i agree. if need ctf, get everything but ctf, if need oddball everything but oddball. swap it out and get a challenge that is kill related or something and suddenly its an even mix of game types. it knows what you need and makes it difficult to get so you use/buy swaps. everything in MP is about spending money then gameplay.
Did you only realise this now 
We kinda have already talked this point to death since Nov 15
343i will probably just go away from challenge swaps with time, simply because the challenge system they have now is much more general