How come you cant remember your childhood memories?

Well, researchers are looking into this, and one of the current ideas is that parts of the brain that are largely responsible for forming long-term memories aren't fully developed in the first several years of life, so what memories you do have from early childhood tend to be very fragmented.

Two primary reasons: The hippocampus, the primary anatomical portion of the brain involved in laying down memories is not yet mature enough below the age of three to consolidate memories as it does when we're older. The other reason is that up until age three is that our language skills are fairly rudimentary. Language is a crucial component of what psychologists call the 'narrative self' or the autobiographical self'.

well apparently S. Freud said that it is child amnesia that happens at around 7 years old. It is necessary as to not remember the traumas one could have experienced in the early stages (Oedipus complex, stage of development, etc) of life which would be dangerous for our mental health. of course it is not eradicated from our mind since it stays in our unconscious mind and that would be why we can have flashbacks much later in life (brought back by a photo or a smell or person etc)
check out his (Freud and his five stages of development) theories and you will understand more.
hope I helped a bit.
have a great day.

I can remember the good and bad ones.

Your mind worked at a different level at that time, with different thoughts and you had very different habbits in your life. Usually when you're a child you hardly even notice it but just like in the dream state you tend to change very much even from one month to the other, not to mention years. The reason why you can't remember is that you were almost an entirely different person back then and you have become someone quite different right now.
An easy way to remember your childhood memories if you wish to is by closing your eyes and guiding your thoughts slowly away from your day-to-day lines of thinking while trying to remain alert and observing and trying to go backwards in time with memories that were first closer to your current time (such as your elementary school graduation or something) and then gradually going back to some events that happened when you were 3 or 4. If you can relax enough and still remain awake then gradually it will all come back to you naturally and with very little effort.

Why would you want to remember falling off a swing?
I believe ,as we get older we get memories that replace the childhood memories.
For instance do you remember the first time you learned that fire was hot?
I'm betting you have a more recent example of getting burn that reminds you not to touch the stove.
I've retained some of the really good stuff of my childhood.A lot of it is feelings attached to a specific happening like when my mother would hug me when I was sick.

because young children have short-term memory and it isn't developed until later.

I guess that is God's way of blocking any bad memories as well!!

i don't know....

I remember mine.

Our brain structure is slightly different, so we may not "know" how to retrieve those memories. Think of it as having a new computer, and trying to get memories off of the 3.5 inch floppy disks. (The thin black ones). You may be able to do it with a lot of practice, but do you really need it. Also, around 2 1/2 our brains kill off a lot of connections that are not being used. So memories llikned to those places may be killed off.