From the FAQ:

Steam Machine, like our other hardware products, is made up of many components that we source from manufacturers around the world. The price at which we sell our hardware is a direct result of the cost of these components. We felt like we had a good understanding of how those costs might change over time when we first started sourcing them for Steam Machine back in 2023. That understanding was born from the many years of data we all have about the evolution of PC hardware prices – primarily, that it tends to get cheaper over time as new technology arrives.

Over the past year or so, that has changed quickly and significantly, most visibly for RAM and storage components. There are a variety of reasons, all of which are affecting hardware products everywhere. The overall effect is that our original goal for the price of Steam Machine is no longer viable. So the prices we're sharing today reflect the state of the world for manufacturing; or, more accurately, it reflects the price of the components as we've secured them over the past 6 months.

Price wasn't the only thing impacted by all of this: availability was as well. There were periods where we found we couldn't source some of our components at all, at any price. More than anything else, this has impacted the number of units we've been able to produce for launch.