I think all of it. It's due to the "I want it, but not in my backyard" syndrome. The incumbent will always have the advantage, unless it really screwed up totally.
The moment you think like this, that moment you gave up your rights to winning.
There are oppositions who won the electorate's imagination. JBJ got his in a by-election. If it is a backyard syndrome JBJ would not have won. LTK, CST and others had been given their chance.
Conceptually it is good to have oppositions. But the implicit assumption is that these oppositions will be able to advance their cause, voice their dissatisfactions and make the PAP more honest than they think they are.
Had LTK and CST delivered? They had 25 years to do that.
Therefore it is not the backyard syndrome that is at work here. Rather it is the lack of competence as demonstrated by the entrenched oppositions. If you doubt this, take a look at the thread on 10 reasons why the PAP must fail. Determine which are the valid reasons. Then take a look at the WP and SDA website, their speeches to see how many of these reasons are voiced. Their websites are controlled by them, not the PAP.
Concluding, it is more helpful to think of the public's reticence to vote for the oppositions as the oppositions inability to demonstrate sufficient competency. Then the question what consitutes 'sufficient competency' will arise.
Though not a political party, the TOC is more competent in reflecting people's needs on the ground. Similarly the use of NCMP is a good alternative to demonstrate to the people that 9 NCMPs can give voice to their needs.
Call it 'probation' before being offered a 5-year contract.
Still, I will not discount the possibility that there will be a significant swing of votes towards the oppositions for the coming Elections, enough to get them a few more seats.
Maybe Singapore's hopes for good oppositions should rest in the new citizens. KJ is effectively a foreign local talent. We need more of such people in the opposition.