The meta is a cruel thing. Not all decks run as many rares and mythics as possible, but higher rarities tend to see more play. In Modern, a deck needs to be able to quickly or effectively enact its gameplan while having consistent answers for as many other common decks as possible. From what I've noticed of your deck, it seems to be the two Arena starter decks melded together, and that's most likely the problem. Modern is a format of efficient threats and answers, where almost everyone runs fetchlands (
Scalding Tarn,) other fast mana sources, and cheap spells. By the time you cast a
Thundering Raiju, the
Murktide Regent deck probably has a
Ragavan, Nimble Pilferer,
Ledger Shredder, and a handful of interactive spells. THe downside of older and more powerful formats is that they're often extremely expensive to get into, and this is why I suggest Pauper if you intend to play paper Magic. Budget decks in formats like Modern can be fun, but if you're looking to win some games, you need to get your inner Spike on and figure out what sees play, and how to counter it.