TeleVue is moving away from Lanthanum glass due to the reduced availability of Lanthanum for glass made in Japan and Taiwan. The PRC controls the world-wide distribution of many rare-earth elements, though this may change. Currently, lanthanum for glass from Japan comes from the PRC to the US and then gets sold to Japan, since the PRC refuses to sell it directly to Japan due to some trade disputes. That has resulted in many companies finding other ways to produce glass with low dispersion and high refractive index (or simply having the products made in the PRC). It's possible to achieve the same thing with more elements or sharper curves, or both. The current Ethos and Delos eyepieces are examples.
Ideally, glass with high refractive index and low dispersion is desirable--the first because it reduces the extremity of needed curves on the surfaces, and the second because chromaticism is desirable to control.
Note that lanthanum in glass is not the only way to achieve that.
There are many kinds of ED (extra-low dispersion) glass in the market, with a wide variety of refractive indices. Which is most appropriate is a function of the glass used in other elements.
I also note that some optical gurus say that ONLY FPL and OK glass is truly ED, while all the others are not, but the market seems to call anything with low dispersion ED, which would include the lanthanum-containing glasses.
For more info, see:
http://en.wikipedia.org/wiki/Low_dispersion_glass and the links therein.