Couple of things.
Production of plastic minis takes tens of thousands of dollars upfront. The cost is in getting the metal moulding blanks engraved, and in the use of high-temp high-pressure plastic injection machines. Once you've put that money in up front, actual cost of production (material and labor) is low, so it makes sense to produce plastic minis in large (thousands or more) quantities to recover the cost at a reasonable price. The upside is large quantities and lower unit price; the downside is massive upfront costs, and inflexibility.
Metal minis have to have masters made, then silicide moulds prepared - manual, therefore time-consuming and labor expensive (need the right skills). But we're talking more like a thousand up front per mini. Casting also takes skilled labor and time, and metal is expensive these days. Also the moulds wear out with too much use, and have to be remade. So while the setup cost is smaller, the production cost per unit is higher. The ability to make modifications though is much increased. So the upside is flexibility and lower up-front investment; the downside is unit expense, and lower quantity production.
The other flip side of metal minis is that IWM only has so many hands, and so much floorspace. While in theory we could be fan-funding dozens of all-new minis every year, that bites into the labour and equipment pool they have to produce other BT minis.
That said, IWM has supported BattleTech minis for decades as a labour of love. We're hardly their major customer - they make much more producing minis for other companies. And they're prepared to keep making minis as long as the moulds last - even minis selling less than 6 units per year. I'm sure IWM and CGL talk, and balance each side's respective needs in their business relationship. I don't want IWM burning out on BT. It obviously makes sense to them to produce the minis available in plastic, to make them available individually, or outside the major investment of plastic production runs, and supports the hobby.
