Why? Well, let me answer with a question.
Why does CA have to join the style Blizzard makes multiplayer games? If I wanna play an arcady RTS I'll play Blizzard’s games or something like LoL. Judging from the game footage, the units all move without inertia, they can turn immediately, just like in Starcraft, or snake for mobile phones for god’s sake. APM rules supreme here.
Next up in CA Total War franchise is Warhammer Total War. I'm pretty sure that game will have same arcady set-up.
Look, I don't blame CA for trying to go mainstream, they probably think these games will sell better - in Arena's case through a system of DLC and microtransactions - but just like Zenimax aspired for Elder Scrolls to go MMO to appeal for a more mainstream audience - as if that was ever needed - it alienates customers who are after the original unique experience.
Just like TESO is a major flop because it tries to make a franchise into something it was never made to be and also because it suffers from severe competition from better and more established franchises (WoW), so is Arena a huge waste of time and money. It will never be as good as Starcraft or the like, so why would people play it (at least for long)? The core players, are the most important for a company like CA to survive in the long run, so why not focus on making games for them? They are loyal. The target audience for Total War Arena will soon move on to better games.
The list of games game-developers have tried to make more mainstream thereby failing utterly includes, TESO, KOTOR Online or whatever the name was, Deadspace 3, Resident evil 5, Sim City, the new Thief, etc etc.
In the end, CA is wasting its money and time on making games that will never be able to compete. Hopefully I'm wrong about both Arena and Warhammer, but judging from CAs recent track record, and especially the footage from TW Arena, CA is losing its way.