Actually a well-gemmed crusader has a higher max then basically any SS. As far as 'superior' seals that is entirely wrong. We have a skill seal and a cripple. Sm has a skill seal and a MELEE SEAL. And if SS are so good and nothings wrong with them, then why are there so few? Also I understand that crus are pricy, but if SS were actually decent then wouldnt that drive strikings up in value due to the common law of supply and demand? Yes, yes it would.
A well-gemmed crusader requires an absurd amount of money. I guarantee that out of those "600" crusaders in the server, very few can go above the highest SS's max.
SS's skill seal is stronger and Cripple is in most situations way more useful than melee seal. Hell, if you cripple someone they probably won't be able to melee at all.(SM melee skill is only better against other SSs if we're looking at the "melee restriction" per se) Great for chasing, kiting and PvE. Let's not forget that SMs in this version are awful because they have no damage on melee seal and no mini-stun on skill seal. And as FinalLegacy said, SM can't compare in terms of speed or DPS to SS.
Let's face it, even if crusaders were awful, the server would still be crawling with them. It's most people's favorite class. Flashy, arguably has the best style in the game, and it's satisfying watching your Crusader progress, it's the class with most visible milestones. Absolutely the most popular class for as long as I can remember (Chuck popularizing them with his slasher build?)
SSs are very reliant on others early on. (Most SSs need taxi in DW till very high level) and that's not something many people enjoy. They're very squishy and the feeling of a cruz popping out and one slashing you at any given time is to many a very unpleasant one.
Lastly, there's skill level to take into account. Though some can argue that SSs are very easy to play as "sealbots" early on, they have a lot of tools at their disposal which require quite a bit of skill (kiting, sealing the right targets, staying safe), so naturally players want to pick a class which is easier to learn. How hard each class becomes to master later on is arguable though.