Because the object of "pli da spaco". "Pli" as an adverb can't take the accusative, and "spaco" is after "da" which never takes the accusative.

If you said something like "Ni bezonas pli grandan kvanton da spaco", you would see the accusative.

Laŭ Duo, oni ne bezonas meblojn. Oni povas sidi kaj kuŝi sur la planko. Estas faĉile. Ĉu ne?

You can use pli directly with adverbs and adjectives.

• pli rapida = faster
• pli longe = for a longer time

When working with amounts, you need da - the preposition for amounts.

• Pli da tempo = a greater amount of time.
• pli da spaco = a greater amount of space.

In certain cases, we can use "more" for both of this situations in English, but that's not how Esperanto works.