Alakazam and Glaceon and Jynx and Raikou have always been available for use in this metagame, as has Blissey. The would-be special threats have had every opportunity to show that they actually are special threats since the metagame started, in the middle of 2007, and began to take shape throughout the rest of the summer and year. If they failed to do so in this timeframe, they failed to become part of the true, Standard metagame, and it's as simple as that.
Jumpman, I'm going to have to interject here. The pokemon listed, especially Raikou are not in the standard metagame because of 2 reasons, and ONE of them IS Blissey. The other goes for their lack of being able to find other niches in the metagame other than special sweeping. For example, look at all of the OU pokemon that have viable special sweeping capabilities:
Salamence
Rotom-A
Latias
Heatran
Jirachi
Lucario
Infernape
Gengar
Azelf
Starmie
Magnezone
Celebi
Dragonite
Suicune
Zapdos
Kingdra
Togekiss
Empoleon
Jolteon
Electivre
Roserade
Porygon-Z
Now I can go down the list 1 by 1 and tell you why each of these Pokemon is OU, despite having viability as a Special Sweeper. There are two main reasons. A) They can BEAT blissey, or B) They have found another role through support. With Alakazam, Jynx, Raikou, and Glaceon, their is no other role. Let me go into detail.
Salamence - No Brainer.. SpecsMence is walled by Blissey and is one of the least common sets (wonder why?).. Has a myriad of physical options
Rotom-A - Can wall or Sweep. Beats Blissey with Sub/Charge
Latias - Calm Mind + Refresh
Heatran - Taunt + FireBlast, Explosion, or Flash Fire Choice Specs. Also makes good support.
Jirachi - Substitute + Calm Mind, can do other shit.
Lucario - Lol.
Infernape - Lol.
Gengar - Explosion.
Azelf - Explosion. Can lead also.
Starmie - OU because it Spins and counters two of the metagames biggest threats, Gyarados and Infernape
Magnezone - Explosion. Traps steels.
Celebi - Special Sweeper is never used because of how much other shit it can do.
Dragonite - see Salamence.
Suicune - Substitute + Calm Mind, Crocune, Pressure.
Zapdos - is relegated to support and tank because it CANT beat Blissey...
Kingdra - see Salamence
Togekiss - Can beat Blissey with Nasty Plot + Air Slash + Heal Bell / Aura Sphere
Empoleon - Beats Blissey with T-Spikes... makes good anti-lead
Jolteon - Baton Passer
Electivire - Cross Chop + Physical sets.
Roserade - lays Spikes / T-Spikes instead of Special Sweeping. In fact, it is never a special sweeper because it won't beat Blissey
Porygon-Z.. the LAST pokemon in the entire tier.. needs extreme lengths to beat Blissey, but it can. Offers nothing besides pure special sweeping.
So you see my point? Every pokemon that is a designated Special Sweeper with no other niches is out of OU BECAUSE of Blissey. You can't say that with Physical sweepers without any other niches because there is no physical equivalent of Blissey... For example, pokemon like Breloom and Machamp are limited in their roles as Hard-hitters and can be played defensively to help with Tyranitar. A physical equivalent of Blissey would restrict them to that role and their usage would drop dramatically...
So I wouldn't be so hard on the four you listed as "not being part of the metagame." They were never going to be part of the metagame solely because of Blissey. Personally, I stay away from special based pokemon as pure sweepers for that very reason. If you look on 98+% of successful OU teams, all the special based pokemon on the team are there for reasons OTHER than special sweeping, or they have a way to beat Blissey and its as simple as that.