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Google is way more than search. YouTube, Cloud, Android, and Pixel are all part of Google.

Because the Google brand is still orders of magnitude more recognized than Alphabet especially in mass market. I would wager a decent chunk of their target audience doesn't even know what Alphabet is

I'm not even sure why they tried to brand a conglomerate holding company anyway.

They didn't. Alphabet has zero branding, barely even a web site. The stock itself is still called GOOG.

Google is much more than that; think Google Cloud, which I think this falls into. Also even hardware (Chromecast, Pixel) falls under the Google umbrella!

Brand identity, I suspect. They're the only folks in the world who are allowed to (legally) prepend "Google" to a service or product, which is essentially free marketing.