Originally Posted by PhiIipz
=> Unique <= Mana Charge: Each time you cast a spell or spend mana, your maximum mana increases by 4 (3 second cooldown). Bonus caps at +750 bonus mana.
It's to prevent the manastacking from getting out of hand and making the items way more effective than they should be. I am suprised you even can start stacking again once your manamune/archangel transformed. This way you can have 3 fully stacked items.
You can have 3 fully stacked items, but the only three champions who might want to consider it are Blitz, Singed, and Ryze, and then they'd have to give up some other important item to do it; probably not worth it, all things considered.
To answer the OP though, it's also to prevent someone from suddenly getting 3,000 mana all at once (ryze would get an additional 450 damage over ~6 seconds or less, Blitz would have an additional effective 1500 hp, and Singed would get 750 more hp). That is, having an additional 3000 mana is obviously something Riot doesn't see a problem with, but getting that all at the 25 minute mark is. And I agree; being able to get that kind of mana mid game would make a number of champions very difficult to deal with.