Darkiddd- What story and themes? It's supposed to be a game of exploration in a new galaxy. Why can't each Milky Way galaxy race be behind something like that? The
Shepard trilogy was a primarily human story of the first Spectre. If Bioware really wanted something fresh and new and they cared about player choice like they claim, they should've given players the ability to choose their race. It worked wonders in Dragon Age Inquisition after all.
It's a game about exploration yes, but you will be looking for a new home for the human race and probably other races (council races and their military that came with you?)
Now let's imagine that in the game you can choose to be a human, krogan, asari, and turian. Now the problem is that every interaction in the game must be carefully planned and developers need to pay attention to what is said and done by you and the NPCs and that it doesn't clash with the personality of the character, who can be human, turian, asari, krogan, and male or female (except for the asari). This ends up making the interactions you have with the people of the game completely generic and makes the plot completely generic. Why? Because everything is done to fit into the mold of an indetermined character who can be many things at the same time. The problem only gets worse if you are planning to make many games with this character, which is what I think will happen.
So when you claim that being able to choose different races for the MC is good because we can define our MC and it gives us freedom and immersion it's actually the opposite case. The only thing defined about the character is the looks and some reference here and there but that DETRACTS from immersion, it doesn't add to immersion. The people who choose to be a human will feel their character outlook and stance is not defined even if they see what their character is (or should be) because the world doesn't acknowledge it. The same with the people who choose to play a krogan, or a turian, etc...
The plot would need to adapt to this undefined protagonist for many games, if the result in Inquisition was poor and this formula works better in that universe then imagine what this would do to a Mass Effect game and maybe a new trilogy with this character. Your companion interactions and romances would need to adapt for this undefined character. Everything would need to adapt to this undefined character, and thus when you need to cover such a wide variety you settle for a bland plot and vague interactions, and the human you chose to play doesn't feel like a human, the krogan you chose to play doesn't feel like a krogan etc...
Making only a male and female protagonist is already a huge effort and asset expense. And it has worked so far for two reasons: the race of both is the same and because we can't compare what Mass Effect would have been with only one gender for the main character, it would have been better and more detailed for the people who want to play that specific gender, but it would obviously suck for the other audience.
So you need to think like the developers and understand that the game needs balance between what is set in stone and player choice, between immersion and being able to influence the world with enough response. With a voiced main character and cinematics where you can choose different things you can't go crazy and offer many options or else the main character will feel bland and uninteresting, which is what happened to the inquisitor. This worked in DA:O because the character didn't have a voice. And works in games like skyrim for the same reason (still skyrim's plot is boring partly because of this)
So if you are going for an immersive experience (And in Mass Effect this is vital) Then you are going to compromise on one race. Unless you want to spend six years of development for each game if you really want to give such variety and still make the character feel alive. And this won't happen and it is completely fine by me because the company can't afford it.
And since Mass Effect is a game made by humans for humans and the majority of people want to play only human or even if there were options they would still play human Bioware decided that if they must settle for one race then they must choose human, it is the most wise and inteligent option for the sales, the plot and the feel of the game. And for the players to bond with the main character, because as someone has already said the aliens in mass effect are humans and human traits with different skins, having them as your companions is one thing, playing as them is another.