The game is broken into three main abilities that either make or break a unit. Movement, attack power, survivability. If you look at some of the best units in the game, you'll see that they are good at all three of these things, or amazing at two of them and have an option for the third.
An Eldar Jetbike: moves 12", boosts another 24" or 36" I don't remember, has four S6 shots with a Scatter Laser, has a cover save of 4+ whenever it needs it and a basic armor save of 3+ and as an added bonus, they're Troop choices so get Objective Secured.
Eldar Wraithknight: moves 12", T8 with an Armor Save and Feel no Pain, has D weapons.
Tau Riptides: move-shoot-move ability, 2+ Armor, 5+ invulnerable that can be a 3+. Options for FNP, S7 AP3 or better guns, options for Skyfire, Interceptor and a few other ways to counter opponents directly.
Space Wolf Thunderwolf Calvary: moves 12", 3+ Invulnerable save, T5 so only doubled out by D and S10, S10 weapons and often helped out by psykers for Invisibility and occasionally ally shenanigans for FNP, Ignores Cover and other cheesy additions.
These are all the cream of the crop, and yes there's others but that's the idea there. So for comparison's sake, let's look at the pillars of Tyranids. Hive Tyrant, Carnifex, Tyrannofex and even Warriors.
Flyrant: 12-18" movement, must be hit by the enemy on a 6 if they don't have Skyfire, 12 S6 shots, 11 hit per average round of shooting, S5 Template with Haywire and psychic powers, doesn't have Eternal Warrior but can't be assaulted with Force in the air.
Carnifex: moves 6", 18" guns. Small threat bubble really. T6, still can be wounded by Bolters, 3+ armor and no way to improve it or get better armor and no way to get FNP other than a psychic power. Dies in one hit to anything with Instant Death like a Librarian's Force Staff.
Tyrannofex: Almost the exact same stats, but 2+ Armor. S10 gun that misses half the time. Instant killed by a Force weapon or D weapon.
Warriors: move 6", S5 three shot guns, options for AP3 in Assault. 3 Wounds. That would be a good unit in Survivability and Attack power but, T4. Instant Killed by a Lascannon or anything above S8. Lascannon aims, wounds on 2+, remove a model. That makes them a bad unit.
Now, that's the individual units in comparison. Tyranids are lacking with every other unit but that Flyrant. So then strategy is the next best thing. 75 point Drop Pods? No point since the units in them would just be inferior. Walking across the table with a butt-ton of threats? Focus fire the only ones that matter and be done with it. 200pts of Guard can and will kill 400pts of Nids if built properly and with a bit of luck. Tau won't need luck. So you can't walk and can't drop in, so you have to do something else. And while every other army but Daemons has the option to put their guys in cheap transports and cross the table, Tyranids don't have that option. We feel every shot and remove models, so we have to plan for that which means more points. If you want a fully healthy Carnifex to make it across table, you need another to die horribly. If you want 15 gaunts to live, you need atleast 25. That gets expensive fast. Infiltrate! Lictors. Now what's going to Infiltrate with them? Genestealers are absolute garbage. Price of a Marine but can't charge through cover without tripping over and 5+ armor is nothing. Literally nothing. They're great in Zone Mortalis but anything without endless line of sight blockers and hallways they are bad, and tournaments will not be Zone Mortalis unless advertised as such. So then, Mawlocs. Mawlocs are great. Ignores Cover, hits in the movement phase. That means nothing the enemy has but Flyers are safe from it. Warp Spiders die, Bikes die, everything dies. A great unit, and nice and cheap.
So the list is now Flyrants, Lictors, Mawlocs. No point in Troops so we take the Mucolid Spores. 15pt Troops. Venomthrope or Malanthrope for Turn 1 protection before the Flyrants fly away and protect themselves. That's the traditional Tyranid list, especially in Tournaments. When you see your Carnifex get taken down by a couple Plasma guns, or when you see your gaunts get systematically slaughtered by TWC or Dark Angels, you just stop bringing them. Biovores are good against Daemons, not good against Marines, so you don't bring them. Exocrine is too slow and dies immediately to Tau, so you don't bring it. Every other choice in the codex fails in one blindingly obvious way or another, so they don't have a place in competitive. Competitive is ruthless and you have to be able to make a list that does just as well against Skyhammer as it does against Imperial Knights and just as well against Eldar Scatbikes and Wriathknights. If it can't survive and win against all of those lists consistently, it's not a good enough tournament list. This is why I never committed to a tournament build. I don't like that side of the game or the limited Tyranid codex.