Why has such a powerful card been placed back on the limited list?
The ways of Konami are not always crystal clear, but consider the following scenario:
Bringing Monster Reborn back will bring balance to the current format. How could that be?
Well, this card would empower those decks that were starting to get outdated (many of them, actually), due to the notable power difference new cards, like Links and Pendulum, are having in the current format.
Monster Reborn was a Staple Card back then in all decks, as it greatly supports basically any card archetype. Monster reborn could be "just" what some older decks needed to lift their game.
With it, those decks can now take an opponent's powerful Link monster like Borreload to even the match up, giving them better competitive level and encouraging players of those decks to become more active players. Also, as older decks had "slower" dynamics (usually taking more time to summon a big monster) this will bring them up to speed so they perform better on today's games.
Now, one may say "but modern decks could also use Monster Reborn, thus giving them further advantage, so how come there is balance?".
It is true that more recent decks could also include Monster Reborn into their build, but realistically speaking it may not be worth it. Many, if not all, modern Archetypes have their own cards that can easily bring back monsters from the GY. Many of them also have "searcher" cards, that can quickly help to build combos and fetch those other cards that bring back monsters.
This means that including a copy of Monster Reborn "just because" may not help those decks, and could even impair them; it could be a dead draw in cases where a card of their own Archetype was needed. In terms of probability this also means that you now have fewer chances of pulling your Ace card (
1/41, when before it was
1/40), and even fewer considering dead draws not giving you your "searcher" cards.
In other words, modern decks already have several alternatives for Monster Reborn, whereas older ones have not. This could benefit the game in general, bringing more variety and balance to the format.