Seriously? Your grasp of singapore politics is a joke. Let me explain it to you in simple terms.
No one listens to the other oppo parties until elections comes along. The noisier Oppo you mention was only allowed to show itself during the campaigning period. People start to do their research and study all the parties once the elections is announced. Prior to that, the govt controlled media makes sure that the viewpoints of the oppo parties are muted and not presented to the voters and the public. Which voter here starts deciding who to vote for in 2012? What about in 2013? 2014? People only seriously started looking at parties only after the date was set for the elections. Prior to this, you have to go to their websites to see what is their platform. Only after the campaigning period starts can the oppo parties hold rallies, book debates, etc and get their message out. They are only given 9 days for this, from Sept 1 to 9th!! Oppo parties have only 9 days to get their message out to the people, whereas the PAP and WP had 4 years since 2011 to get their message out. The time is too short, People are not able to digest all the infor from all the parties in time. Up until a few days before the campaigning period, the oppo parties were still trying to trash out who will run where. The voters did not know until maybe 2 weeks before the actual vote who was actually their oppo party.
Contrast this with the US. The next US elections is in Nov. 2016. Yet all the debates have already started, all the candidates are already on national TV and radio and making their policies and viewpoints known. The US voters have months to study all the candidates. People have a very good idea of what Donald Trump is about. They also have a good idea of who and what the Democrat and Republican presidential nominees stand for. I can guarantee you 100% that if the campaigning period was 3 months, there is no way the PAP would have these numbers. The fence sitters need to study the oppo party in their constituency. If they don't have the comfort level they will go to PAP. But there was not enough time to make this connection.
The other factor is the PAP has the resources to track the 2011 voting in minute detail. They know which ballot boxes in which polling station had more oppo votes then PAP votes. They re-draw the boundaries to make sure they break up any oppo stronghold that could have developed. Coupled with the fact that sample polling has a psychological affect on voters that have not voted, and in the end you have the results you have. Several elections ago in the US, I recall one presidential candidate was winning by a landslide in the East Coast. because the East cast their ballots earlier due to the time difference, poll results were announced in the East even before people in the West Coast got to vote. when people in the West coast saw the run away results, they simply voted with the winner. No one wants to go to the office the next day and say they voted for the loser. Its just the human psychology. The PAP employed this when they used the sampling vote results. They may have been faked, but when people who have not voted find out that the PAP is winning comfortably, they simply vote the PAP too, because they think that voting oppo will not matter due to the PAP lead.
These are just a couple of the reasons. There are many more, Its not as simplistic as you say.