[ciphol]

Because the only people who feel the personal need for a "free speech" platform are the ones whose views are considered odious elsewhere.

If people with non-"odious" views end up at such a platform, they will quickly notice the unusual concentration of "odious" views, and generally find it uncomfortable and leave. Thus there is a steady increase in the prevalence of "odious" views until they are near-universal on the platform.

[dundarious]

And the people who run the "non free speech" platforms (i.e., all the popular ones) are operationally "opposed" to free speech primarily in the sense that they want a pleasant platform for advertisers, and hence must stifle the stuff that's unpleasant to advertisers. There is some personal ideology at the top of these organizations, but it is extruded through a profit seeking filter and often barely survives operationally intact.

I don't say this to excuse their actions or say that this filter is good, but that seems to be the fundamental mechanism.

[dredmorbius]

Not strictly true.

Those whose focus is overtly offensive and oppressive speech, however ...