I think it's safe to say there are various reasons why Raikkonen is 'quite a stretch away' from Alonso.
Alonso is planted well into Ferrari for a couple of years now. The team (atleast what it used to be) clearly enjoyed
working with Alonso, even if the 'ferrari dictatorship' won't 'allow' certain 'demands' from Alonso to happen.
In other words, the 'design facility' of Ferrari will build the F1 car, and that's what you'll get, that's what you'll drive,
and for the rest of the season, it's your and the mecanics problem.
Alonso is used to this environment by now, and has adapted well through years and years of F1 experience, ups and
downs, championships and just-out-of-reach.
Furthermore, Alonso is top-class F1 driver.
Raikkonen left Ferrari, ditched and dumped, and imho, if i look at interviews, specials, and whatever of Kimi at maranello, I see a man that 'doesnt give a shite' about whoever is around him. Example; Kimi crashes in Monaco, and instead of going back to the team has a drunk in his boat while the race continues. Even though that was at Mclaren, it just shows the level of commitment. I can see both the funny side but also the respectless side of that. Outcome is that there is simply less 'motivation' from the team (mechanics, etc.) towards Kimi and vise versa - something totally different to Alonso.
Kimi ain't a bad guy, he just makes it hard for himself.
Now kimi finds himself back at ferrari after enjoying his 'freedom' at Lotus/Renault, back into the Ferrari 'camp', in a F1 car that might be one of the worst Ferrari's designed ever, both aero and engine.
Like said before by other members; Kimi doesn't adapt as well as Alonso. Kimi is struggling with the car and is clearly getting frustrated. His crash when he went off-track and slammed his spinning car into massa imho is a sign of that, that was quite un-kimi like. I think his 'personal life' is interfering more aswell. He can't reach his former 'level', i'd say because he lost motivation after 1. winning the Championship (been there, done that) and his entire personal life and experience changed. So kimi back in 2007 is not the kimi we see now.
In the meanwhile, Alonso maintains his calm, is still after his 3rd World Championship, has the patience, and seems not 'effected' by circumstances as much as kimi does.
That being said, i do not believe however Kimi is as good as Alonso in an average car. Kimi is seriously fast, but it must be said yes, when the environment isn't that good, he can't reach his level, whilst Alonso seems to be able to cope with any environment.
I'd even go as far as to say Kimi is faster then Alonso, when in a perfect environment (unfortunately for Kimi, a perfect environment is rare so he'll miss out to Alonso). If I look at Kimi at Spa, overtaking balls-out in the most impossible places,
I see a man with incredible courage, speed and confidence.
Alonso however keeps his calm and is far more 'tactical', and can overtake in perhaps less 'bold' manouvres, but technically more correct moves where he'll be able to put his car in a way the other driver can no longer hold his position.
Long story short: Alonso is better in less then perfect circumstances, Kimi is amazing in perfect circumstances.
And let's face it; Alonso has 2 driver titles versus 1 of Kimi. So statistically, its a battle won always by Alonso.
BTW, If Hamilton didn't have tech issues, or if the the stewards actually penalised BMW and Williams for the fuel
' irregularities ', Kimi never would have won that championship.
Next year probably will be a fairer battle between the two, if Alonso will remain at Ferrari (which i'd doubt seriously,
and I think Ferrari signed Kimi because they knew, know, or plan to ditch Alonso anyway, and must have a WDC in one
of the cars).