Progression that people can see
Ambiguity is read as a no.
People leave when they conclude they will not advance, and that conclusion is frequently reached on incomplete information. In the absence of a clear picture of what advancement requires and whether it is available, capable people assume it is not and look elsewhere, sometimes while the organisation intended to promote them.
The remedy is specificity rather than encouragement. What does the next role require, what does this person currently lack, what would demonstrate it, and roughly when might it become available. That conversation is uncomfortable because it may end in an honest no, and an honest no is far better than a vague yes.
Where there is genuinely nowhere to go, which is the reality in many small organisations, saying so plainly earns more loyalty than implying otherwise. People can plan around an honest limitation and they leave abruptly when they discover a concealed one.
Lateral development is undervalued as an alternative. A person who takes on a different kind of work, a project outside their function, or a meaningful piece of responsibility frequently stays several more years without any change of title.
The other reason a technical track fails is that it is usually created without changing who is in the room where decisions are made. A senior practitioner with equivalent pay and no influence has a title rather than a path, and capable people identify that distinction quickly.