The difference is that a LCS team picked them up, it is not that they are better or worse :D
Speaking seriously, the skills you need in a team environment are partially different from the one you need to succeed in soloQ. In solo Q you should aim to have as much carry power as possible on yourself because that's the fastest and more reliable way for yourself to win game, in team play you got to do what is necessary for your team to win, which could mean filling a different purpose on your team
Basically achieve challenger can show that you have the mechanically skills, but from that point there is much more you got to do to succeed in a team, and that's why some player completely fail in LCS despite being soloQ gods (Woolite, Gillius, Mash and other exmples in NA)
Paradoxically even some masters (or Diamond 2 lel) players would be better than some LCS players, cause they have those skills to make up the lack of mechanical skills (and often it isn't even a matter of mechancics, but a matter of having lower peaks on some super carry soloQ champions that can more easier led you to climb the ladder)
P.S. on a side note, that's why i was aaginst the magnitude of changes tone to challenger ladder in the CS qualifier process, play in an amateur/semi-pro team before getting to a CS/LCS team is an experience that can greatly improve players and can help to distinguish earlier in the development process the players capable to translate their soloQ play to team competitive environment