Because almost every input costs more than it does anywhere else. The ships are ice-strengthened and purpose-built for a route they can only sail for about five months a year, so the capital has to earn back over a short season. Guide teams are far larger than on a normal cruise, because the landing model needs a lot of qualified people running Zodiacs. And the rules cap how many passengers can be ashore, which limits how much you can spread fixed costs across.
Two things soften it. Expedition fares include more than you expect, so the sticker price is closer to your true cost than a mainstream cruise fare is. No port charges, no end-of-trip taxes, no overweight luggage fees, and meals and every excursion are in.
And the price varies more by timing than people assume. The same ship on the same route in March can cost a third less than it does in January.
If the number is the obstacle, tell us your budget honestly rather than shopping in silence. There's usually more room in the late season and shoulder departures than people expect, and we'd rather find you a trip that works than have you decide the whole thing is out of reach.