It's plausible that Proton does not care about their Tor service, but there may be another reason: The new version relies more on Javascript code than their old version, and a Tor user is more likely to browse with scripts disabled than a regular user. Proton may be holding back the rollout of the newer version until they have tested it more without Javascript. This is only a hypothesis, and I came up with it just now; take it for what it is.