The need for higher resolutions, higher refresh rates, increased color depth, high-quality uncompressed audio, and less compressed video (chroma subsampling), increased the need for increased bandwidth. HDMI 1.4 was limited to 10.2Gbps, which was enough to push 4K video at up to 24 Hz, while in comparison the latest standard, HDMI 2.1, supports a bandwidth of 48Gbps, and is able to support 4K @ 120Hz, or even 8K @ 60Hz.

The chart below quickly summarizes the differences in features between HDMI versions.

Source: HDMI Forum