She has a high pick rate because she is very effective in countering double shields. The high winrate is a byproduct of having a very high winrate in previous seasons and because not many people know how to properly deal with Sym.
Furthermore Brigitte and Symmetra’s winrates did drop in previous seasons. Brigitte’s winrate dropped once her shield and shield bash got nerfed. Symmetra’s winrate dropped once 3.0 hit. Only now its rising because Brigitte is a better healer and Symmetra is very good against shields.
Also data was not collected during the beta season so pickrates and winrates are currently all over the place.