Why does 'lowdown' mean 'the true facts and the relevant information'?

It is an AmE usage:

lowdown, the n. (orig. US) privileged information, intimate details, ‘the inside story’; thus adj.

• 1901 [US] W. Irwin ‘Prologue’ in Love Sonnets of a Hoodlum n.p.: On the low down, dear Mame, my looty loo, That’s why I’ve cooked this batch of rhymes for you.

(GDoS)

Its origin in the above sense is not clear, as suggested by the Phrase Finder:

It could that this is just a variant of the age old saying 'to get to the bottom' of something; ie to get to the roots of the matter.

and the American Heritage Dictionary suggests:

The lowdown on:

• The whole truth about something, as in We're waiting to hear the lowdown on what happened after we left. This term uses lowdown in the sense of "the basic or fundamental part." [Slang; early 1900s]

from which the expression:

get the lowdown (on someone or something):

• To receive or find out specific or comprehensive details about someone, something, or some situation.

(The Free Dictionary)