Are there any English dialects where you isn't simply used for any possible reference to the second-person? Y'all is often considered as a rough approximation of ustedes and vosotros. Is this as far as it goes? Any others?
There are still a few spots of dialects left in the northern parts of the UK and Scotland where a different second person singular pronoun, the modern development of the older standard thou, has persisted in various forms, mostly pronounced [ða] or [ðə], sometimes [ðɛ].
As John Lawler says in his comment, there are many more variations than just y’all; y’all just happens to be the most common one. I don’t think there are any English-speaking areas that don’t have some kind of ‘double plural’ to mark a specifically plural you, be it y’all, yiz, youse, y’uns, yous’uns, all y’all, or even the almost comical alls y’alls’uns, which by my count marks its pluralness no less than five (!) times.
And what's so different about English that causes it to not develop in the way that Spanish does?
Nothing. Nothing whatsoever.
You see, every single language on the planet develops in its own, unique way.
- Some develop an intricate, elaborate, and confusing system of pronouns (Japanese, for example, has an intricate system of pronouns that depend on both the gender and social status of, as well as the speaker’s attitude towards not only the speaker and the addressee, but also the physical antecedent of the pronoun);
- others develop very simple systems (standard Mandarin—if we exclude various dialectal or extremely formulaic forms, as well as artificial orthographical choices—has no case, gender, or numerus distinctions and makes do with just three in total, one for each person);
- and others yet never develop a system of personal pronouns at all (Vietnamese, for example, uses conventionalised nouns rather than actual pronouns).
So it’s not that there is something ‘different’ about English that makes it not develop like Spanish, but rather that they’re different languages and therefore, quite unavoidably, develop in different ways.
As it happens, I would say that they have developed in very similar ways. They have both gotten rid of nearly all case forms throughout the language, but maintained a greater level of case distinction in the personal pronouns than elsewhere (Spanish maintaining three cases in some pronouns, English managing only two); and if we limit ourselves to at least some voseo-speaking areas, they have also both gone through a shift where the second person plural became used to signify politeness, followed by the singular being perceived as impolite and subsequently all but dropped entirely; and the original plural being then used mainly as a singular, from which a separate plural must be secondarily formed.
If you ask me, that’s quite a parallel thing to do—though it must be mentioned that it is in no way a rare thing for Indo-European languages to do. Rather the opposite.