Why do most of us get the chicken pox or measles once?

Once you have had it you build up an immunity.
That being said it is more than possible to have them more than once.

Even if you've had chicken pox, as an older adult, you can get something similar called shingles.

From webmd --

Shingles (herpes zoster) is a viral infection of the nerve roots. It causes pain and often causes a rash on one side of the body, the left or right. The rash appears in a band, a strip, or a small area. Shingles is most common in older adults and people who have weak immune systems because of stress, injury, certain medicines, or other reasons. Most people who get shingles will get better and will not get it again.

Shingles occurs when the virus that causes chickenpox starts up again in your body. After you get better from chickenpox, the virus "sleeps" (is dormant) in your nerve roots. In some people, it stays dormant forever. In others, the virus "wakes up" when disease, stress, or aging weakens the immune system. It is not clear why this happens. But after the virus becomes active again, it can only cause shingles, not chickenpox.

You can't catch shingles from someone else who has shingles. But a person with a shingles rash can spread chickenpox to another person who hasn't had chickenpox and who hasn't gotten the chickenpox vaccine.

Even if you've had chicken pox, as an older adult, you can get something similar called shingles.

from webmd --

Shingles (herpes zoster) is a viral infection of the nerve roots. It causes pain and often causes a rash on one side of the body, the left or right. The rash appears in a band, a strip, or a small area. Shingles is most common in older adults and people who have weak immune systems because of stress, injury, certain medicines, or other reasons. Most people who get shingles will get better and will not get it again.

Shingles occurs when the virus that causes chickenpox starts up again in your body. After you get better from chickenpox, the virus "sleeps" (is dormant) in your nerve roots. In some people, it stays dormant forever. In others, the virus "wakes up" when disease, stress, or aging weakens the immune system. It is not clear why this happens. But after the virus becomes active again, it can only cause shingles, not chickenpox.

You can't catch shingles from someone else who has shingles. But a person with a shingles rash can spread chickenpox to another person who hasn't had chickenpox and who hasn't gotten the chickenpox vaccine.
I not only have had a couple of cases of shingles(once in school as a teen,then chicken pox a second time as an adult,when my son had his,then shingles again as an adult,not more than 7 years ago:( )

It can cause many problems for an adult(chicken pox),but thankfully I was/am too crazy to suffer from much of them:p

Seriously though, to the op's question,as curlyben said, immunity and time play a part on having the illness more than once.The virus,as I understand it,stays in your spinal fluids and when your immunity system is lower than the threshold of your ability to fight the infection,it can resurface as many different symptoms,shingles and chicken pox just being a few of them.