[2021] Why isnt a Champ like Zyra played mid ?

For what it's worth Zyra is a competent mid laner. Just because something isn't popular doesn't mean it's weak.

On her initial release, she was (and somewhat still is, just to a lesser degree) skewed toward being a stronger support than mid laner and in general, support players tend to like playing Zyra so she remains popular in that role. But again she's a functional mid laner and if you like her, go for it.

Generally, Zyra beats mid-ranged champions. She can keep champs like Annie, Orianna, and Brand out of range and use her plants to take lane control.

She has okayish peel, so she's actually fine against some assassins like LeBlanc and Zed. Some others are just really tough to beat back like Yone.

She has very limited gank-assist options. Poke champions like Vel'Koz and Xerath play out of range of her plants while clearing minion waves as well or better so those are also weak matchups.

Overall, she has some OK assassin matchups and generally good mage matchups.

In general, all solo lane champions have some rough counterpicks. The existence of bad 1v1 matchups does not make champions bad. Orianna is the globally most-played mid laner in pro play despite having some very rough counter-picks like Corki and Ekko.

Well her very median win rate would disagree with you.

Roughly half of mid laners are stronger than her, yet half is not most. About half of mid laners are also weaker than her, but people still play Orianna and Syndra in solo queue.