Why is term life insurance so much cheaper than whole life or universal life insurance policies?

Term insurance may be cheaper than whole life or universal life for a few reasons.
 
First, term life insurance is temporary - for a certain number of years. If you outlive the policy, coverage ends. Whole life continues for your entire lifetime, if you pay your premiums. So, the chances of paying out the death benefit on term insurance is less than for whole life insurance.
 
Also, people are living longer, so this may add to fewer death claims for term insurance policyholders.  
 
Finally, there is no cash value build up within most term lfie insurance policies, you pay for the insurance only. Whole life polciies usually have a cash value investment type of portion within the life insurance policy. So, you are paying for the investment part of your policy, as well as the insurance with whole life insurance.