Because that is how Bluetooth is specified. Bluetooth is primarily used for audio, where high throughput is not the most important parameter. Expected Bluetooth throughput is less than 2 Mbps, see more here https://www.bluetooth.com/blog/exploring-bluetooth-5-how-fast-can-it-be/
WiFi on the other hand, has peak throughput above 1 Gbps. So your observations are as expected.
Have you measured the power consumption with a kill-a-watt (or similar)?