This is most commonly for one of two reasons: Your bag is bigger than the maximum carry-on size allowed by the airline. You're boarding toward the end of the boarding process, and the overhead bins are full.

Airlines force passengers to "gate check" their bags primarily because overhead bin space is a finite resource that often runs out before all passengers have boarded. Modern aircraft are designed to carry hundreds of people, but the bins were historically not sized to hold a "maximum-sized" carry-on for every single seat. When a flight is full, the bins usually reach capacity around the time the middle boarding groups are called. Additionally, airlines are under strict "Turnaround Time" pressure; if passengers spend too long struggling to find space or rearrange bags, the flight will be delayed. To ensure an on-time departure, gate agents will proactively tag bags for the cargo hold once they estimate the cabin is full. You can often avoid this by purchasing priority boarding or owning a co-branded airline credit card that grants you an earlier boarding group, ensuring you are among the first to access the empty bins.