Because it is not pleasant. Many people get kicks out of everything, but most experiences I've lived with DMT were not the most pleasant. There are occasionally times of sheer emotional pain! These are the best chances for growing, of course, but it is not pleasant. There are many drugs that can produce pleasurable feelings: LSD, Mescaline, 2C-B, and so on, but I would never put DMT on their side. Most people I know always take a deep breath before starting and *always* ask: "do I really want to do this again?"
Because it may frighten you. And if you try it often enough you WILL be afraid. How do you cope with your fear is up to you, and also a chance to grow, but many people avoid fear and for such reason do not touch DMT ever again.
Because DMT may not be strong enough. I know I will be slammed for this statement, but never mind. DMT is very strong, but there are other stronger molecules, like Salvinorin-A or, particularly, 5-MeO-DMT. Both will convey different but much stronger experiences. Those who are in for the ultimate experience will find greener pastures elsewhere
Also remember that most people who try DMT (in whatever form) do not do it again. What you find here at the nexus are the hard-cores, the ones who really are in for the insight the molecule produces, so this is probably the last place to ask!