Because they serve two different purposes.
The Stack Exchange Network - or Stack Overflow, really - was designed with the intent of giving people a place to ask questions about a specific topic. It was designed in a way to focus on objective questions based on facts. Opinion-based questions simply don't work well on the network outside of very specific guidelines (see Good Subjective; Bad Subjective).
Stack Exchange is about questions with objective, factual answers. We’ve been crystal clear about this for as long as I can remember, even back to the earliest, pre-beta days of Stack Overflow. It’s right there in the standard Stack Exchange FAQ:
What kind of questions should I not ask here?
Avoid asking questions that are subjective, argumentative, or require extended discussion. This is not a discussion board, this is a place for questions that can be answered!
There's a really great explanation there about why this decision was made and how to make subjective questions work in some cases - some sites lean a lot on the good subjective concept, though still eschew the bad subjective.
Meta came later but it was built on the same technology.
We have questions and answers and comments... but meta is specifically about deciding what's best for the site (or sites). This is, inherently, subjective.
- What should be on topic or not?
- What features should be implemented?
- What should we do in X situation?
To steal from the blog post:
These sorts of things on the right side (opinion, judgement, preference, belief, rumor, suspicion) are essential to a meta site (hopefully not rumors... but...). It's what Meta is built on.
It's allowed because it must be else the Meta sites fail to serve their functions.
These discussions should be supported. Hopefully the discussion is more than just "this is how I feel, so let's do that". Much as the Good Subjective; Bad Subjective blog post recommends, discussion and subjectivity are best served when supported with either personal experience or a reference from somewhere else... and a few other things.
The bullet points to writing a good subjective question (without explanations) are:
- Great subjective questions inspire answers that explain “why” and “how”.
- Great subjective questions tend to have long, not short, answers.
- Great subjective questions have a constructive, fair, and impartial tone.
- Great subjective questions invite sharing experiences over opinions.
- Great subjective questions insist that opinion be backed up with facts and references.
- Great subjective questions are more than just mindless social fun.
I've written a lot of posts on Metas - both here on MSE and on several child meta sites and these hold out. The questions and answers that get the most attention and support are the ones that follow these guidelines. When they fail, they get little attention or they get closed... and sometimes, if they fail point 3, they get very heavily downvoted, as these tend to read like rants.
The rules are more lax on Meta, this is absolutely true. But that's because we're using the same back end for a completely different purpose than what it was designed for. This can make true discussion questions pretty difficult to follow. Questions get dozens or hundreds of comments. In some cases they attract many answers, each often with their own pool of comments where following conversations can be very complicated.
Maybe someday we'll have a new meta design that is better suited to the format here. The current design works well for support and bug questions but less so for feature requests and discussions... or we'll continue to make do with what we have.
It's worth noting that the term "meta tags" is a bit of a false friend. The required tags here on Meta are not what is meant when we're talking about "meta tags" on main sites. On main sites, meta tags are tags that are based on something other than the question itself.
One example (which aligns with the required tags) is a tag with the purpose of classifying the type of question being asked. An example on main sites might be beginner. This tag doesn't tell us anything about the question itself, more about the person asking it. It's subjective in and of itself. It's a bad tag, so we avoid it.
Another example is tags that define a large percentage of the subject matter of a site. On Movies & TV, they have blacklisted tags for "movies" and "TV" because nearly every question on the site would have that tag. It's useless and too generic.
You can find more info about meta tags in Robert's answer here.