Metagross
Metagross has become another Tyranitar: Unique typing, bulky, but with bad movepools. Metagross has slightly higher attack that Ttar, but even worse moves.
Psychic and Flash Cannon are even worse (same damage but longer duration) than Stone Edge, which is already a bad move. Earthquake is slightly better than those two (but not a good move either), but lacks STAB.
Because of this, Metagross also can't be the defensive monster it should be, because of only having 1-bar charge moves.
Answers
Not to be rude, but you aren't trying. I haven't played since Zapdos was a raid boss and I know your account name very well. At this point if you don't have a few level 30 plus Machamps, you are clearly putting your resources towards other things, which is a mistake on your part if you still struggle with Blissey.
It's all about time. The slower the defender goes down, the more time its owner has of noticing and feeding. The slower the whole gym goes down, the higher chance that a member of that team not in the gym gets there and feeds until they can add their own pokemon in place of one you kicked already. The slower your own defender goes down, the more time team mates have of getting there and adding defenders.
The mechanics are a bit more complex than just "anyone can take down any defender duh".
In my opinion, Metagross has a few small strengths over Tyranitar, though, for the most part this post is accurate.
1. Safer dual-typing. Tyranitar is a wall against certain types, people like to forget that because it has a double-weakness. However, Metagross has 3 less weaknesses, and no double weakness, as well as a couple double resistances. Fire-types are the only Pokemon to be a real threat against them.
2. Dual-type capability. Metagross has some pretty awful moves, yes, but, unlike Tyranitars albeit somewhat viable Bite/Stone Edge situation, Metagross has quick move/charge move synergy for both of its types, even if they are poor move choices, as well as having STAB on both types for mixing defensive movesets. I haven't seen a whole lot of Pokemon that resist Steel and Psychic simultaneously, apart from silly Houndoom, and that isn't even to mention its 3rd charge move option.