Validation of BST using Inorder Traversal recursively

Question

consider a binary tree in c with structure given below

struct node{
int data;
struct node* left;
struct node* right;
};

And this is the code for to check if a given tree is Binary Search Tree or not

int isBST(struct  node* root){
static struct node *prev = NULL;
if(root!=NULL){
    if(!isBST(root->left)){
        return 0;
    }
    if(prev!=NULL && root->data <= prev->data){
        return 0;
    }
    prev = root;
    return isBST(root->right);
}
else{
    return 1;
}
}

I want to know why we made the node prev static and when does this part of the code section violate the criteria for BST

if(!isBST(root->left)){
        return 0;
    }