1) super short answer: Midi Clock doesn't have the ability to allow slaved device to be nudged (in a way useful to the art of DJing). PRO LINK does allow for this ability (since it was designed as a communications protocol for timing/DJ uses). [kind of unsatisfying answer but mostly correct.]
2) even shorter answer: see page 20 of the DJS-1000 manual. [it took me a few years of DJS-1000 use and re-reads of this page to fully unpack what I think this page means for answering questions like "why can't Midi be nudged on device X?"]
3) longish answer: PRO LINK allows the sequencer of the DJS-1000 (in Player Mode) to synchronize to the MASTER player (a device playing a rekordbox analyzed track) via TEMPO (BPM), BEAT (quarter note or 1 step button on the DJS sequencer), or BAR (64 steps equals 4 Bars in the parlance of the DJS ). if the DJS-1000 is in SYNC mode with a master player, and you use NUDGE, the SYNC light will start flashing meaning the DJS-1000 is now synchronized via TEMPO only (same BPM just slightly different play head position relative to Master?), opposed to being 'locked' to the Master player's BEAT/BAR and set to same TEMPO.
MIDI allows the sequencer of the DJS-1000 to synchronize to an external Midi device (MASTER Midi device) via Midi Clock, which for the DJS-1000 means via TEMPO. but since the DJS is Midi Clock slaved via Tempo, temporarily changing the tempo isn't possible and if it was it would simply go back to the same sync state you wanted to nudge out of [poorly explained answer]
Maybe that helps in thinking about the 'whys' of the DJS?