This is a story I put together from several sources. I can't verify it but it seems reasonable.
Back in the 60's, JBL had a government contract for some models in the D series. Government contracts demand that you give them the very best price, and that you never increase the price. I think this is intertwined with what was at the time called "Fair Trading" a rule governing retail sales which ment you could not get JBL's at a discount or the dealer would not be allowed to sell JBL's. Conrad talks about this in one of his interviews posted on the Sunn Shack site. He bought 10 JBL's from a dealer in Portland and JBL dropped the dealer when they found out. The only way JBL could increase the price was to discontinue the D series and replace it with the K series. Same speaker, just slightly different cosmetics. Now when the government came calling, you would think JBL wrote into the contract that they could increase the price if the price of Cobalt went up which it did. The E series has ceramic magnets that don't use Cobalt. They came out when the price of Cobalt got so high that sales of the K series started to fall off. FYI, the ElectroVoice SRO series and Altec speakers like the 421A had ceramic magnets.
While this seems reasonable, and I'm certain that the cost of the rare magnets materials played a big part, In the case of the JBL's I think it probably had a lot to do with the power handling capacity. The D140 was a bass speaker, the "King of the Hill" in it's day, when and 75 watts was a lot of power.
Remember when a Crown DC300 was a "monster" amp, and when the 350+ per side BGW's and Phase Linears were just
unbelievably powerful?
Of course now days sound systems often run into the tens of thousands of watts, and people regularly run 500 - 1K watts in their bass rigs, sometimes in 2 or 3 rack spaces, and those old AlNiCo speakers just won't keep up the pace. It's a shame, nothing sounds quite like those old D series, but by todays standards they are dinosaurs, just like the old power amps.
There are still old school AlNiCo drivers being made, but almost exclusively designed and meant to be used in guitar applications where a 50 watt watt amp will generally get the job done, unlike the required power usually percieved to be nessesary for bass guitar.
The SRO was AlNiCo and switched to ceramic magnets, and then the same speaker became the EVM series, and I think pretty much still exist as the "Force"? series.
Altec is a great example of power generally (and rather quickly) increasing, they went from the 420 series with AlNiCo magnets to the 421 series with ceramic, virtually the same 3" aluminum VC, same cast aluminum basket, and upped the power rating from a whopping
25 watts to 150 continuous program.