Every word has a form (what it looks like) and a function (what it does). For example, the word "We" is a personal pronoun in form and a subject in function. The word "All" is an indefinite pronoun in form--"Indefinite" means it does not directly refer to any particular individual or thing--and as for its function, it has many: It can be a noun, an adjective or an adverb.1) We the people want to talk to you.
2) All the people want to talk to you.
If "All/all" occurs with the preposition "of" (e.g., all of), it functions as a noun, and when a pronoun functions as a noun, it's called a substantive noun; That is, in the phrase "All of/all of", "All/al" substitutes for a noun, so it's called a "substantive". If "All/all" occurs with a noun, it functions as an adjective:
1. All of the people... (Substantive Noun+of)
2. All the people... (Adjective+Noun)
The reason "All/all" is called a noun in 1. and an adjective in 2. has to do with structure: Nouns occur with "of", and adjectives occur with nouns. That is, "All" is a pronoun in 1. and 2. in form only. In function it's a noun (1) and an adjective (2).
In short, given, We the people versus All the people, "We" refers to "the people"; the pronoun "We" and the noun "the people" are one and the same, whereas the same cannot be said for "All". "All" is not a personal pronoun; "All" does not refer directly to "the people". They are not one and the same. "All" tells us the amount or quantity of people: It qualifies the noun "the people", and that's what indefinite pronouns do: they qualify nouns.