Ignoring the fact that you can do stuff with "flexible" resin moulds you cannot do with rigid plastic moulds: in days of yore (and remember this is days of yore, your modern mould mileage may vary) manufacturers had to match the price of the moulds against the expected models sold.
Resin was cheap to make the moulds and cheap to mould but the life span of an individual mould was short. The cost of the mould was a significant element of the cost of the individual moulded item(s).
For a tale of woe try "The Tale of Andy's Dragon":
http://www.heresyminiatures.com/shop/blog/White metal moulds were more expensive to make (requiring specialist processing and the masters should not contain plasticine - as used by a youthful Perry Twins to the surprise of a Citadel mould maker). The spin moulds used now costing even more than the Prince August style ones. Their lifespan varied (depending on the amount of spikyness). The cost of the mould was a proportion of the cost of the item(s).
As an aside, I remember reading about the use of cuttlefish "bone" as mould material for white metal. Health and safety tips, one - don't, two do not give it back to the budgie after use.
Plastic moulds were (and are) specialist engineering, in the UK most "independent" gaming plastics use moulds made by Renedra. They often last the lifetime of the product (I hope that the Kitmaster originals now sold by Airfix and Dapol are not using the original moulds but you never know).
The mould cost per item was insignificant over the lifespan of the mould, but you had to be able to sell enough to cover the cost.
Technology has moved on and resin moulding has improved.
These resin tracks offer a way to test the market for the improved track. It might be that a multi-slide plastic mould is viable, or some other method allows the tread detail missing from the existing technology. It also allows an improvement to an existing product without the expense of a new set of moulds.
As Elias mentioned, the plastic kit market is full of after market upgrades and this is an in-house variety. The Tiger is a nice kit, it has caused few issues but the improved tracks are a luxury item.