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Its hard to explain without talking about how your immune system works.
When you fight a new infection, your body needs to learn what antibodies to produce to fight the virus. To do that it first needs to find an "antigen" from the virus. An anti gen is like a part of the virus it can identify, and so it can make anti bodies for fighting that virus.
Your body has something in it called a T cell. T cells remember what anti gens your body has encountered before so your body can quickly make anti bodies if the virus ever returns.
So, if you get measles, your body will remember what measles viruses look like, and you cant get measles again because if the virus ever shows up, your body will notice and respond by producing anti bodies against measles.
Why dont the vaccines work against covid? Well they kind of do, to the extend that your T cells will react to the vaccine and remember what the covid antigens look like. But they dont work, in that, for some reason that I honestly dont know, your body does not react fast enough to stop a covid infection when a covid virus returns several months later.
This isnt particular to covid. The flu and many common cold viruses are the same way. If any of these viruses infect you, you get the proper T cell response, and for several months after your body is pumping out anti bodies to attack any infection. But after that your body just kind of chills out, and it has a hard time producing a big anti body response next time the viruses come.
Again, I dont know why these viruses are different, but I wonder if it could be that these viruses multiply very fast in your body? So its not that your body is unusually slow to react, but instead these viruses will reproduce a lot before your anti body response kicks in.
