Obviously winning a title requires things to come together, and in 2008 things clearly weren't happening for him. After the crash in Monaco, he had three consecutive races where he was comfortably ahead of Massa in Canada, France and Britain. He threatened to win all of them, should have taken at least 26 (old money) points but for poor luck and strategy errors. He ended up taking 13 points. So he wasn't maximising the opportunities for big points. Then he had three poor performances, tied into qualifying struggles in Germany, Hungary and Valencia - although in Hungary he actually gained ground. By this point momentum was all with Massa in the team. Obviously he lost points in Spa. It was another error, but hard to be harsh as everyone was making mistakes in those conditions. Then he was poor in Italy. Fast in Singapore and could well have had a Ferrari one-two without Piquet's unfortunate mistake. Would have had a decent chance of winning Fuji but the Mclarens forgot to go round the first corner. Gave a position to Massa in China. You can easily tot up an extra 30-40 points if he'd been maximising everything that year. Obviously all title contenders dropped a lot of points that season, save maybe Kubica. Ultimately his qualifying record was going to be tough to overcome in a title fight. But the perception was probably compounded by him being hit by misfortune in some of his better performing races. Plus both Ferraris were solidly awful in the rain. In hindsight, the writing was probably on the wall after round one - mechanical failure in qualifying, a strong start and a chance to salvage some very good points, but a trip through the gravel after a safety car and it all unravelled.
You're quite right. Raikkonen's 2008 season was downhill from Monaco onwards. He was actually solid but from then onwards it was some amount of bad luck and a couple of mistakes (which they all had that season).
Monaco - Drive through penalty after to failing to have the tyres attached on the car by the team before the three-minute warning before the race, dropping him from 2nd down the order and then the crash with Sutil
Canada - Hit by Hamilton in the pitlane when fighting for the win
France - Reliability dropping him from 1st to 2nd giving Massa the win
Silverstone - Was faster than Hamilton and gaining on him until the team decided to keep him and Massa on the Intermediates while the rest of the grid bar a couple went on to Full Wets, resulting in the famous Ferrari spins
Valencia - Hose issue that Massa had in the pits and lost a podium
Spa - Had a slow puncture at the latter stages but Ferrari kept him out resulting in big gains from Hamilton
Singapore - Race was ruined as he was queued behind Massa when the fuel hose got detached, tried making up for it and crashed
Fuji - After the first corner shenanigans by both McLaren's lost places and had to cover ground
China - Moving over for Massa
In what was a strange season for the top guys, he was the one that was worse off with things outside his control. I think his season is frowned upon due to Massa having the better luck and Kimi making a couple of errors, but overall he was faster of the two and would likely have had more wins.