savedInstanceState always null (Activity A to Activity B back to Activity A)

Question

This is a Java language project for Android application using Android Studio.

I go from Activity A to Activity B and then back to Activity A. Nothing is transferred from Activity A to Activity B, but on the way from Activity B to Activity A there is. Here is code taking you from Acitvity B to Activity A:

@Override
public void onClick(View v){
    Intent main_intent = new Intent(PeriodicTableActivity.this, MainActivity.class);
    main_intent.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
    main_intent.putExtra("Element", String.valueOf(v.getTag()));
    startActivity(main_intent);
    overridePendingTransition(R.anim.fade_in_fast, R.anim.fade_out_fast);
}

On the way back from Activity B to Activity A, I want to use Bundle savedInstanceState to restore a HashMap<Integer, ElementInfo> instead of parsing web URL that fills the HashMap in the opening of the Android application.

if (savedInstanceState != null)
       elementInfoHashMap = (HashMap<Integer, ElementInfo>) savedInstanceState.getSerializable("ELEMENTS");
try {
    InputStream input1 = getAssets().open("periodic_table.csv");
    elementInfoHashMap.putAll(pullElementURLData());
} catch (IOException | NetworkOnMainThreadException | ExecutionException | InterruptedException e) {
    e.printStackTrace();
}

Here is how I store my HashMap<Integer, ElementInfo> when I leave Activity A.

@Override
public void onSaveInstanceState(Bundle savedInstanceState) {
    super.onSaveInstanceState(savedInstanceState);
    savedInstanceState.putSerializable("ELEMENTS", elementInfoHashMap);
}

My issue is that whenever I return to Activity A from Activity B, my savedInstanceState is null. Does anyone have an clue why savedInstancedState is null when returning from Activity B to Activity A?