What limits wireless bandwidth?

The upper limit on bandwidth is the maximum data rate defined by each standard. That data rate is chosen to reflect the capacity of the physical medium and modulation techniques. For example, a single 2.4 GHz channel can carry frames modulated with the Barker code at 2 Mbps. That same channel can be used with Complementary Code Keying (CCK) to carry frames at 11 Mbps or with Orthogonal Frequency Division Multiplexing (OFDM) to carry frames at 54 Mbps.

Clearly, different modulation techniques can pack more information into the same "space" to increase throughput....

Proprietary high-speed WLANs use variations on these standard techniques to provide higher bandwidth, either by changing the way data is transmitted or by bonding together more than one channel.

Note that data rates are not the same as throughput. Data rates represent the speed at which frames are transmitted/received. But those frames include data, control and management traffic, as well as errored frames and "dead air" when no frames are waiting to be sent. As a result, application throughput is much lower than the negotiated data rate.