Cause you suck bro, ehehehe git gud…
OK sorry, but we need more info, posting “Why am I bad?” doesn’t really help. Are you doing raids, M+, PVP? What is your talent setup? Are you running any priority helper addons? Do you actually understand the classes you are playing and what their skills do?
For example you say you are bad at tanking, if you wanted to run a DK, are you pressing Death Strike the minute you lose 10% health, or are you using it to it’s full effect?
“Physical damage and heals you for 25.00% of all damage taken in the last 5 sec, minimum 7.0% of maximum health.” The more damage you take over 5 seconds, the stronger the heal is.
Same goes for BrM with their stagger bar and purifying brew, if you use that 3 seconds into combat you may only clear like 10K damage from your stagger, whereas if you wait as long as you can and build up your stagger bar as much as possible you clear a ton more damage, still 50% of the bar, but clearing it when it is higher gives you more, so you can last longer, get your celestial brew built up faster, than you can setup talents to make purifying brew go on CD faster so you can stay alive longer.
It’s the same with healers and DPS, you need to understand how their spells work and when best to use them, like you cannot be spamming flash heal as a HPal, you will run out of mana super fast and it’s not really an effective spell. Spamming chain heal on shamans is also bad, ripetide > chain heal does a lot of healing, but it also uses a lot of mana, you can use your totems to best effect to supplement you raw healing. Druids can place HoTs on everyone, that slow healing over time is quite effective at healing minimal damage, thus saving your mana for the big hits and having more rejuv makes your regrowth heal higher to further maximise your mana.
This list goes on, read your spells, read guides on how those spells work to best effect, try to understand the buttons you are pressing and how they all interact with each other, don’t just go “Oh no, I got hit, time to death strike” and wonder why your self-healing is garbage.