Because its a synth, not a drum or sample player.
Retrig on an Elektron only retriggers drums (AR/MD) and/or samples.
OT/MD/DT/AR/M:S = drums/samples
The Monomachine is a great illustration of this divide.
Its BBOX drum “machine” is the only machine inside that has retrig. Because it is triggering samples/drums.
None of the synth machines inside the Monomachine have retrig. You can’t retrig the FM machine or the Virtual Analog machine, the same way the Analog Four / Keys don’t have retrig either.
Quite simple, really.
The Digitone has more in common with its synth brethren than its drum machine brethren, since it is a synth.
Ratcheting/rolls/etc are rhythmic FX, which is why you find them present in drum machines more than you do synthesizers.