Because it's counter to the MMO genre.
That's part of the game, killing the boss and hoping to get the loot.
It's the random chance, the feeling of reward, that's a core part of MMOs.
Good or bad, that's how the vast majority of the genre has worked with "endgame" loot.
Plus, your OP doesn't exactly prove there's a problem and doesn't actually address much; it sounds like "kill a boss X times and pick loot", but only once?
But, let's say your method goes into play, and is added as supplementary to current loot (random drops).
By the 3rd week, let's say your guild cleared heroic fully each time.
Now, everyone in the raid, along with whatever may have dropped in those 3 weeks, now picks one item from any boss of their choice.
What happens next week? Do they just get to keep picking or does the points system reset?
I would assume the points reset because otherwise you get a free, specific item each week from that point on, in addition to random drops and trades, which seems like a bit too much.
If it does reset, then every 3 weeks you get a specific item, assuming you killed the boss in question 3 times.
It just seems a bit, I dunno. Much.
Maybe with your system they remove the Great Vault in its current incarnation (which I think would be best, but that's a diff topic) and instead replace it with a vendor that allows you to buy one eligible item?
I'm trying to visualize what you want from your brief 2 sentence OP and it seems like it needs a whole lot of work, thus why it feels silly.
Now, something I might support, given the statement that there's a loot problem (something I don't see, but I'll take others' word for it) would be something like this:
Bad Luck Protection per Boss.
Everyone has a flat 20% chance to get loot to start.
Kill Boss A and get no loot, now you have 40% chance next loot-eligible kill.
Get no loot then? Ups to 60%, and so on until the 4th kill with no loot, giving your next kill 100% chance of loot.
If at any time you do get loot, at either 20% or 100%, it resets to 20% and you start fresh again.
You don't pick what you want, but you are guaranteed to get something.
I think something like this could still keep a measure of surprise and excitement in the loot rewarding system, as opposed to window shopping your leggo bow from Sylvanas because you killed her 3 times.
Sure, you will get items you don't want sometimes, them's the breaks, but you can trade/shard/vendor them and aim for them again.
There's also chances that your guildmates will get what you want and not need it, increasing loot that flows between the raid.
Even this feels like it's a lot of gear flowing around, with a guaranteed "something" every 5th kill (if not sooner), but maybe a good compromise?