English Translation
All this is for my personal use.
Why isn’t “alles” capitalized here?
Because it’s a pronoun, not a noun. See: alles - Wiktionary, the free dictionary
Don’t get fooled by the “Das”. Not everything that follows must be a noun that needs to be capitalized.
In this case “das” stands by itself, and what it refers to is unmentioned. “alles” is just an addition, you could just omit it in this sentence - exactly like in english.
Basically, it’s “(all of) this (<stuff>) is for my personal use”.
A counter-example where “Alles” is actually a noun would be:
Du bist mein Ein und Alles. - You are my everything. (“my one and all”)