Understanding Society User Support

I am using 5 different waves (waves 9 to 13) and have decided on the weight m_indscui_lw as I watched the video on choosing the weights and think this is my best option. I am using middle aged-adults for my project. But I have ran the frequencies on this weight for my data and it says there are approximately 2000 participants with a weight of 0. I am just wondering why this is the case. Thanks very much