Yeah... But you could make fields for the others too, if you want, and you can make your fields List-Type fields so you could add multiple values to your custom [Choir] field. So, you could have a [Quartet] field, or whatever to go with it, and all of those could accept multiple values.
Likewise, if you do want to "group them all together", you can just make your own [Orchestras] field, which is a List-Type field, and just use that instead. MCs built-in fields aren't "special" in any way. They're just some suggested presets.
The only reason not to make your own is for interchange with other applications that might recognize the [Orchestra] tag, but not MC's custom [Orchestras] or [Choirs] tags. But, even if you do care about "interchange" with some other application you use, and they do support the regular [Orchestra] tag, they're not going to support multiple values there, and import it as a semicolon delimited string, and so you're back to the same issue with those other applications too. So that's... Meh.
List-Type fields are handy. But they also do have a substantial tagging-workflow downside:
When I'm tagging using the Panes, I can very quickly spot an "outlier", and fix it. Maybe Philharmonic-Symphony Society of New York is spelled right on most of my files, but on one or three of them, it somehow got typed as Phliharmonic-Symphony Society of New York. This will stand out like a sore thumb in a pane showing [Orchestra] (there will be two of them right next to one another), and it is super simple to fix. All you have to do is click on the badly named pane to select the files, and then check the box on the pane (probably right next to it) where they should be. Presto-whammo it is fixed.
But list type fields are additive, not replacing, when you tag, so now I have files tagged as Phliharmonic-Symphony Society of New York;Philharmonic-Symphony Society of New York. To fix it, you have to also uncheck the "bad" item after you check the good one. It is minor, but it can make things sloppy in a big library. And that's not the only way they're "slower". It is more steps to tag them in the Tag Action Window, and unwieldy to tag them inline in a View.
The answer isn't always List-Type fields, which is why many of them are not that by default.
But, you can make them. You did make one, you're just storing it in MC's string type field (and using the wrong delimiter).
None of this is to say one way or the other how I feel about them actually changing [Orchestra] to be a list type field. I don't really care. But I'm pointing out that it is No Big Deal if you want it that way, they don't have to change it for everyone. Make your own [Orchestras] field. Switch your Views and columns and whatnot over to using [Orchestras] instead of [Orchestra] and move your existing field data over to it. Done.
I should note, if you ever do this again, use a semicolon to separate your values, not a comma. MC uses semicolons (that's how those List-Type fields work, they're just strings that are semicolon delimited). So, if you had used semicolons instead of commas, you could just make your [Orchestras] field, and then use Library Tools > Move and Copy Fields tool to move the data over to it, and you'd be done.
As it is, even if they do make [Orchestra] into a list-type field, you're going to have to change all those commas into semicolons.