Thank you sir.
We have limited starbases so mines could be the same. As a rationale, make them phased-antimatter mines that require exact and careful placement in a gravity field and are essentially small robotic ships packed with stealth cloaking, sensor and targeting gear--plus the containment for a large amount of antimatter and a reactive mass.
In fact, to make them really interesting, damage could apply to ALL parties--friend or foe. Another reason to justify small numbers of them in a system and adds another layer of strategy for both sides and makes the role of scouts in detecting them all that more vital.
Plus, the amount of small mines needed to blockade planetary space approaches a scale that is completely absurd.
You could racially differentiate mines by giving them abilities that activate within their range--reducing speed of nearby ships, preventing phase travel, nanite damaging hull, etc, etc.
In fact, let them be "black hole bombs"--creating a brief singularity when they detonate. They could use the same negative-repulse the DS black stars does to suck in ships and then have the detonation (rapid evaporation of the unstable singularity) occur closer in.
The use of black holes would merit restrictions all parties would agree to. No one wants a black hole dropped on a planet and no one wants to accidentally drop one on their own world.