One reason is that Wyoming was a relatively new state, having only been admitted to the Union in 1869. This meant that it had a less established political and social structure, which may have made it more open to change. Additionally, Wyoming was largely a frontier state at the time, with a relatively small population and a high degree of social mobility. This may have created a more egalitarian society, in which women were more likely to be seen as equal to men.
Finally, the women's suffrage movement was particularly strong in Wyoming. The territory had a number of active suffragists, including Esther Morris and Louisa Swain, who worked tirelessly to secure women's right to vote. Their efforts were successful, and Wyoming became the first state in the US to grant women the franchise.
While there may be other reasons why Wyoming was called the most progressive state on the issue of suffrage, these are some of the key factors that likely contributed to this designation.