Why was Maxx "C" banned from play?
Well, surely Konami had their own reasons for doing so (my impression is that this was done to favor newer decks that special summon more, or perhaps many decks were abusing it) but I think you already found one of the main reasons why:


  it can help people turn a duel around


Indeed Maxx-c can turn a duel around, specially when used on early/mid game when it is most disruptive. 

Like you said, when you see your opponent activate a Maxx-c you are faced with a difficult choice: (1) either keep summoning to ensure building your game and strategy, but give your opponent a huge hand advantage, or (2) stop summoning to prevent giving your opponent hand advantage, but basically delay your strategy a whole turn by refraining from finishing your combo. 

As you can see this is a dire choice, as in both scenarios you have few or nothing to win, hence why this card could have been banned.

Now, as you experienced, Maxx-c is less effective on late game. This is because on that moment it doesn't matter anymore if you give your opponent hand advantage, as chances are that if you successfully carry out your combo you will win right there. In these situations it is usually better to ignore Maxx-c, summon and ensure your victory that turn.  

So, there probably are other reasons for Konami to ban this card, but in my experience Maxx-c has always been a powerful card since it came up; with the current format it has become even more powerful. 

You say that this card could "easily" be countered (although to be honest there aren't that much cards to do so, and the ones that do are really specific, like Debunk), but that would force players to include such counters on their main or side decks, possibly affecting their game strategy. So restricting or banning it sounds reasonable to me.