In this community (and in the entire MC fandom), about everyone seems to think that Mojang is being deliberately lazy with the mob vote for the sake of engagement. It's just constant complaining about Mojang supposedly being incompetent, and most of it makes no sense.
Now, to be clear, I don't like the mob vote. It turns the community against itself, sets people up for disappointment, and puts unrealistic expectations on Mojang for the sake of extra engagement, and I'd like to see it gone. However, I think it's unreasonable to ask Mojang to just add all three and deliver an update of the same size.
Also, I'm not a modder. I have some skill with programming, but as far as Minecraft goes, I'm just some guy who likes playing and talking about the game.
Now, to those of you who think that modders are doing in less than a week what Mojang does in several months, let's take a look at the differences between Mojang and modders when it comes to implementing mobs.
Firstly, Mojang has to implement prototypes of the mob, test them, and tweak accordingly to make sure the feature is well designed. To be clear, I'm not saying that modders don't care about game design, but the community expectations for many mods (particularly proof-of-concept ones like these mob vote mods) are much lower. As long as a modder implements the penguin half decently, tons of people will be happy with it and go "mojang bad, this modder added the mob in 2 days." Meanwhile, there's a lot more pressure on Mojang to get a feature right before release.
Additionally, mob vote mods don't really get updated with, well, all the other stuff that's added to Minecraft afterwards. Mojang isn't just adding penguins and calling it a day; they're adding penguins, then an entire update on top of that, and then future updates as well. Thus, they need to make sure that any code they add won't cause problems down the line.
Another thing that nobody seems to consider is that modders are typically developing for just Java Edition. On the other hand, Mojang needs to develop a feature on two different editions of the game, each of which is in a different programming language and has different underlying code. And because of Bedrock Edition, Mojang needs to add the mob on several different platforms, including mobile devices.
Mojang also has the concern of bugs & performance. I'm not saying that modders don't care about performance and bug-fixing, but expectations are often lower, especially for these mob vote mods. Again, they can get away with just implementing the feature in game, regardless of if they do it to the standards placed on Mojang. And if you're just showcasing the mod in a video, you barely have to care about performance and bugs at all! It just needs to look nice on camera.
Oh, and remember when I mentioned that Mojang is developing for many different platforms? Well, Mojang needs to make sure the mob performs well and is free of bugs on all those different platforms. In two different editions of the game. Fun.
And lastly, since Mojang is a full development studio under a corporation, any feature that they add likely has to go through several layers of approval. They aren't an individual modder who can just add whatever they want, do some minimal testing, and release it to the public.
When people say that Mojang should be able to just add all three mobs because a modder can do it, they completely ignore the completely different situation Mojang is in. A modder can make a buggy, inefficient, half-way decent implementation of the three mobs on just Java Edition, and vast swathes of the community will act like they're doing hurdles of Mojang.
And also, this whole idea of Mojang being lazy and not wanting to work on the game just… doesn't make sense? People act as if Minecraft is being developed by a whole studio of people who don't like or care about the game, but that's obviously not the case. Do you really think everyone at Mojang wants to limit what they add in an update and deal with the constant whining that they aren't doing enough? If they could add more features, they would be doing that—it would only help them. If they aren't, it's because the realities of game development prevent them from adding 20 mobs in an update.
So please, if you want to complain about the mob vote, fine, but don't place unrealistic expectations on Mojang to add all three mobs when that just isn't feasible.