Why there are data bit lower than 8 data bit of uart
In the past Baudot code was popular 5-bit code used with telegraph machines. The idea of being able to shift between letters and figures gave it coverage of uppercase letters, numeric and other common punctuation symbols above the usual total of 32 you'd expect from a 5-bit number.

ASCII is a 7-bit code (although often now transmitted as 8 bits regardless) so that explains why that was common. I'd never heard of six bits being used but presumably in the past if it was worthwhile to cover five, seven and eight bits handling six bits added no additional complexity, but more likely is Michael Karas' answer in that regard that six bit encoding was also popular as well.