The core issue was that 8GB sticks of DDR5 memory in the laptop-friendly SODIMM form factor became unobtainable by the time Valve came to secure the quantities needed for the project. Here's Aldehayyat:
"It was quite literally impossible to buy 8GB [sticks], at least at the quantities we're looking to buy mostly because everybody wants higher storage, and that's more profitable, so all the capacity shifted to that."
That meant the firm had little choice in switching to a single 16GB stick of DDR5-5600, as these sticks were produced in greater quantity. That does mean that the Steam Machine is essentially paying a performance penalty for the decision, though Aldehayyat notes that the two configurations are "pretty comparable" in actual games, less so in CPU-specific or memory-specific benchmarks.
In our testing on Steam Machine and elsewhere, that bears out. If you're GPU limited, as you are likely to be when targeting 4K resolution in many games, then there's no difference due to the single-channel memory setup employed by the Steam Machine. However, there are a growing number of CPU-heavy titles, especially RPG or simulation titles like Crimson Desert, Baldur's Gate 3, Dragon's Dogma 2, Factorio, Cities: Skylines and so on, which can easily show CPU differences - even at 4K.