This question is tricky for me, since one perspective is that a racing-style
inline skate _is_ a kind of skate rollerski -- one with a better-integrated
ski / boot / binding system. An inline skate is a rollerski that has been
optimized for fun, maneuverability, safety, speed, and fun on pavement.

Why are inline skates so much faster than rollerskis?

Actually I think some rollerskis are faster than some inline skates.

I think the biggest factor for speed is the material and design of the
wheels. Other things being equal, wheels made out of hard urethane plastic
will have far less rolling friction than wheels made of softer rubber.

Mount soft Carbonics wheels on an inline skate, and it will be slow. Put
hard urethane 100 mm wheels on a rollerski, and it will be faster.

I'd guess that the next most important factor is the number of wheels. I
haven't studied rolling friction, but I can imaging arguments for why having
the weight of the skier/skater distributed over more points of contact with
the pavement yields lower rolling friction. I recall that one model of fast
rollerski has three wheels per ski.

Seems like more points of contact per ski helps with maneuverability too --
lack of which is my biggest objection to rollerskis.

I'm guessing that the reasons that most rollerskis have only two wheels a
(1) to avoid becoming too much heavier than snow skis; and (2) cheapness.

But maybe there's some other reason?

Ken