Probably released on the same day they go public to generate maximum publicity.

They should be going public within the next few weeks and that would be a very short interval between releases. (And despite the chatter I'll be very surprised if they delay the IPO; it would be a massive PR hit and make it even harder for them in the future).

My speculations: maybe this wasn't as much of an improvement over 5.1 as they were hoping, so they're saving 5.5 for the next major update (possibly "Model 4" from the METR report). Or, maybe capacity is constrained so "5.2" is a subtle encouragement to stick with Opus unless a task really needs the increased intelligence.

They are most likely saving it to one-up Astra 6.2, which is coming this week supposedly.