In halo's early days, parents still forbade they're younger kids from playing violent shooting games. Now, those kids are parents, and they're 14 year old kids have been playing shooters since they were really little. The chaotic, just have fun, explosions everywhere type of game that infinity slayer is, appeals to this generation of short attention spanned people. You see it with a ton of games, not just halo. Every game spoon feeds you now adays.
Infinity slayer lets you have fun even if you aren't a great player. Not fun in the 'oh i'm good at this game' sense, but fun in the 'oh I suck but I can still get kills with this randomly dropped incineration cannon!' That's all fine and dandy, but to echo what others said, that gets old fast. Sure, when you're just messing around with your buddies, infinity can be exciting.
Some people don't have fun like that though. Some people have fun playing in a game where you can anticipate things the enemy team is going to do. Where you can form strategies based on what weapons you think or know the other team has. 'Oh shit, we didn't get the rockets first, so that means they have em' as opposed to 'I have no idea what weapons the other team has' I know in halo 3, and even in reach, I had a few buddies I played with. We tried to play competitively when we played. We were after the win. Infinity slayer ruined that though because it no longer became about us controlling a position, and remember weapon spawn times to make sure we got the power weapons. Each one of us had or strengths and weaknesses and so we'd spend our time in the lobby, once we knew the map, coming up with some plan of action at the start. That was fun. With infinity slayer, that doesn't happen. Doesn't matter how good we hold an area, we never know when someone on the other team is gonna get enough assists and distractions to get himself an incineration cannon.
Everyone votes for infinity slayer because they're looking for 3-5 minutes of explosions and chaos instead of 5-10 minutes of strategy, skill and a bit of challenge.