I think (but I'm not certain) that DT derives its memory layout from dfhack, rather than attempt to work the entire thing out separately. It used to be that when a new release came out, you could use a scanner built into therapist to get the memory layout. Or at least, use it to get a start at getting the memory layout, assuming it hadn't changed too much. But it didn't always work well. So I'm assuming that Splinterz just got tired of duplicating the effort that was also being done for dfhack and decided to piggyback on their work. So now therapist isn't usable until someone gets the new memory layout for dfhack done, and then Splinterz does some extra work to convert it to what therapist needs. Presumably this work is far less than generating a layout from scratch. (Someone who actually know what goes on is more than welcome to chime in here)
This still results in therapist being usable with a new release far earlier that dfhack, because there is usually more to do with dfhack besides just identifying the new memory layout. All the dozens and dozens of dfhack scripts and plugins need to be checked out and fixed if necessary. And the dfhack devs seem pretty conservative about actually releasing new dfhack versions.
If you are willing to compile dfhack, you can get a version that works on a new version before they release an alpha (but you take your chances on it, because it is pre-alpha, which means possibly broken). You just have to monitor their git repo and see when they get their memory layout in. Or you could wait until a therapist release comes out, which would mean that dfhack's memory layout is in.

So far I don't think they've felt confident enough to do a real, non-alpha dfhack release since 42.XX came out. So therapist is way ahead in that aspect.