The reason dates back to 2005 when Congress passed the Real ID Act. According to a release from the Department of Homeland Security, the 9/11 Commission recommended the act, which lays out security standards that must be met when producing and issuing IDs. Eventually, only individuals from compliant states with acceptable IDs will be able to board a plane, enter federal facilities, or enter nuclear power plants. The date for the TSA to stop accepting non-compliant state IDs is January 22, 2018.
Some states are already compliant, while several others have been granted extensions. DHS has a color-coded map available online that allows U.S. residents to see where their state is at in the process. Green states, including Texas, Colorado, and Nevada, are already compliant. Blue and yellow states have either been granted an extension or are under review for one; these include California, New York, and Illinois. Selecting an individual state from the drop-down menu will reveal specific information about its current status.
As Forbes notes, there are other acceptable forms of ID aside from a passport, and they include a resident card/green card or a military ID. However, if you have neither and are a resident of a non-compliant state, you might want to look into getting your passport now. It can take four to six weeks to receive a passport from the date one applies, while expedited ones (which are more expensive) take two to three weeks.