Looks like you need the "quiet part" said out loud:
Chances are, the company was fishing for (or at least wouldn't mind) VC investment, which requires things being built a certain (complex and expensive) way like the top "startups" that recently got lots of VC funding.
Chances are, the company wanted an invite to a cloud provider's conference so they could brag about their (self-inflicted) problems and attract visibility (potentially translates to investment - see previous point).
Chances are, a lot of their engineering staff wanted certain resume points to potentially be able to work at such startups in the future.
Chances are, the company wanted some stories about how they're modern and "cloud-native" and how they're solving complex (self-inflicted) problems so they can post it on their engineering blog to attract talent (see previous point).
And so on.
Yes. Exactly. The company wanted to be "modern" in terms of tech stack and they kept getting buried in the thought that using serverless would keep them cool. The PM was also close friends with the CEO so everyone blindly nods to him.