Quoted from DakotaMike:
I seem to recall Charlie saying that Spooky has to pay something like $100 per playfield for the high-quality wood they use now, while companies like Stern only have to pay $10 per playfield for theirs. (due to the softer and lower-quality wood that they use)
Of course this leads to the question, why doesn't Stern just raise prices by another $100 and get rock-solid playfields? One easy explanation could be, that it's simply not how they think. Big companies would rather use the cheaper wood (since it's "good enough"), raise prices again anyway, and then pocket the difference. Another explanation could be that maybe Stern can't get the quantities of higher-quality playfield-wood that an operation like theirs would need. Since they're producing thousands where others are making hundreds.
I don't know, I'm not an expert on any of this stuff.
It probably comes down to the availability of the higher quality playfields.
Meaning, in the quantities Spooky needs, 500 playfields a year, is probably easier to source than 14,000 playfields a year for Stern. Just a guess.