Because you delete all the items in your code!
You should try to match your criteria before delete it.
SPWeb web = SPContext.Current.Web;
SPList list = web.Lists["MyList"];
if (list != null)
{
for (int i = list.ItemCount - 1; i >= 0; i--)
{
if( list.Items[i].equals("your criteia ")){
list.Items[i].Delete();
}
}
list.Update();
}