What helps to explain why some issues make it onto government agendas

What helps to explain why some issues make it onto government agendas and receive much discussion and media coverage and others do not? Do you think that most issues on local, State and national agendas are proprlled there because of the political and economic interests of legislators (e.g., pork-barrel projects), because of the power of interest groups, or in response to the needs of the public? Why do you think so?