What's preventing aircraft manufacturers to record every single possible value in FDRs?
I'm not an expert on FDRs, but I do have experience of hardware and software development in a regulated market (medical devices), so might be able to offer some insight.  Here are a few considerations that occur to me:


Development cost:  Adding storage to a FDR isn't like installing new DIMMs in your workstation.  As well as the design work, the changed device must undergo new verification and validation cycles, the supply of the new components needs to be assured, and the manufacturer must continue to support existing products as well as a new one.  The V&amp;V may well include accelerated-life testing and other expensive activities; it's unlikely to be just a few days' work.
Customer demand: Aircraft operators require an FDR that satisfies regulatory requirements; it's a "tick-box item".  Consequently, that's the requirement that aircraft manufacturers demand of their FDR suppliers.  If there's no demand for extra storage (either for an extended loop time or for a greater number of sensor inputs), then there's no money to be made by offering that as a feature.
Interface limits: I don't know enough about FDRs specifically, but the input interfaces may well have a bandwidth limit which precludes providing more readings without providing further connectors (which then has a big impact on development costs; see above).