I have another question. It seems just to me or is true that the step sequence Midori Ito did at the 1988 Olympics in her SP was particularly difficult compared to the other ladies at that time?
I'd have to take another look at the programs to offer an opinion on that question. Give me a couple of days to get around to it.
I liked very much the step sequence on just one foot that Irina Slutskaya did at the 2002 Olympics in her SP. Do you know if other skaters did also this kind of step sequences in the past?
Examples of one-foot step sequences I can think of:
David Liu, short program to "The Mission" that he used at 1991 and 1995 Worlds and possibly in the intervening years -- he also did a non-jump version of this program at in the Interpretive competition 1991 Skate Canada, and he had a different interpretive program to Gershwin tunes at 1992 Skate Canada that also included a one-foot step sequence
Brian Boitano, "Elegy" program that he used in the exhibition at 1994 US Nationals and in some other shows and maybe pro competions around that time
Alexander Abt, Boys II Men exhibition performed at 1995 Skate America and probably elsewhere
Surya Bonaly, 1995-96 short program
Dmitri Dmitrenko, 1997 short program to electronic music he helped compose, called the "Short Game Program" (this was a circular step sequence all on one foot), and I think he also had one-foot sequences in 1998 and/or 99
Galina Maniachenko, several short programs including 2000-01
Irina Slutskaya, as you mentioned
Unfortunately we won't see it anymore.

Why COP doesn't reward it? Isn't it more difficult?
Just the fact of maintaining and regenerating speed without pushing to the other foot is difficult, and it's also difficult to change rotational direction on the same foot.
These skills are specifically rewarded in the levels for ice dance step sequences, in which a one-foot section with at least two different kinds of one-foot turns for each partner adds to the level, and also in the synchronized twizzle sequences doing both twizzles without a change of foot and only one or two turns or edge changes in between can also add to the level. But not doing a whole step sequence on one foot, because that limits the number of other skills that could be included.
The level features for singles (and pairs) step sequences don't specifically include extended sections on one foot as a feature.
The features for singles step sequences are:
1. Variety (complexity for Level 4) of turns and steps throughout (compulsory)
2. 4 changes of skating or rotational direction
3. Modest (full for Level 4) use of upper body movement
4. Quick changes from steps to turns
Staying only on one foot, it's possible to have 4 changes of direction and to have upper body movement.
It's also possible achieve "variety" (3 different kinds) or "complexity" (4 different kinds) of turns, but there won't really be any steps at all, since steps involve changing foot. Maybe the initial push onto the foot that's going to be doing all the turns would count as one step, and a hop in the middle could count as another depending how that would be defined, but there would be no other opportunities to fill the variety of steps requirements without doing any chasses, cross steps, toe steps, mohawks, choctaws, etc.
And of course it's impossible to show quick changes from steps to turns without taking any steps.
Also it's easier for the skater to meet rotation changing and upper body features if they change feet than if they don't, so there's no real incentive not to.
If officials want to encourage one-foot footwork skills, which seems like a good idea to me, they could add it as an additional feature in a couple of possible ways:
-Similar to the dance features, make it a feature to include a section on one foot that includes at least 2 different kinds of turns and at least 360 degrees of rotation in each direction
-Make it a feature to cover half the length of the pattern on one foot (from the end to the blue line in a straight-line, or a full half circle in a circular sequence, and define it either of those ways for serpentine) -- then it would still be possible to show the variety/complexity of steps and/or the quick changes in the other half of the sequence to earn the first or fourth features
-Make it a feature to do the whole sequence on one foot. Obviously that would preclude the features that include steps, but the difficulty of covering the whole ice would make up for it. Maybe it could count as one feature just to cover the whole ice on one foot, and an additional feature to cover the whole ice on one foot while including at least 4 or 5 different types of turns (threes, brackets, counters, rockers, twizzles -- could also include toe pirouettes and figures-style loops -- in the long program also one-foot single jumps such as loop, walley, one-foot salchow, one-foot axel, or inside axel)
There used to be a feature about varying the speed (not counting stops) -- this is possible although difficult on one foot, so restoring that feature would give one-foot sequences another feature option to make up for the lack of steps.
Looking at David Liu's one-foot sequences from the early 90s, he used such a good variety of skating skills just on one foot that there really should be a way to define the features so that he would have earned at least level 3 for those sequences, or could have done so with minor tweaking.