Simple. The Batte for Middle Earth series was developed on behalf of and published by EA. EA no longer owns publishing rights for the Lord of the Rings games, which means they are no longer authorized to publish any games bearing the LotR property. The rights have been transferred to WBIE, who in turn are not authorized to distribute EA's property.

The chances of these games ever being released again - or any of EA's Lord of the Rings games, which includes The Two Towers, The Return of the King, The Third Age and Conquest - are close to zero.

Sadly, some opportunistic motherfuckers have decided this automatically makes the game "rare" and have chosen to demand exorbitant prices for used copies and even more exorbitant prices for new copies. Rise of the Witch-King, by way of exception, has actually become rare by virtue of the fact that it originally didn't sell particularly well.

The game is not old enough to be demanding that kind of money right now, so if I were you, I'd look around on eBay and in thrift stores - there you'll most likely find a reasonable deal.