I agree with many of the comment here. Here's my two cents worth (which includes a number of which include prior comments)
There are probably multiple ideas floating around that led to this decision
First, bringing everything under one brand makes sense. Running two separate airlines that were supposed to be the same, doesn't make sense. Silkair has been a thorn for many a *A frequent flyer and this allows SQ a chance to retake the high road on service/product.
Second, differentiating budget from full service The merging of Scoot and Tiger meant that they now have have a large budget fleet, and there is a risk that people lump SIlkair with the budget option. And given it is more expensive (as a full service option) people would get disappointed if they expect SQ level service/product for the price, or they might be confused as to why it is more expensive.
Third and most out there. The arrival of high speed rail will be a game changer.
There were numerous reports that Kuala Lumpur - Singapore was the busiest air route in the world. I suspect that SQ realises that when this route comes online (supposedly in 2026) they will see a precipitous drop in the number of passengers who would prefer the ease of the high speed rail. Rather than fight it with it's full service flights,it seems more likely that SQ will use its budget arm to compete. There will always be passengers who are sensitive to price, so they would use budget.
I think the timing also matters here. Most of the time, (Correct me if I'm wrong), when carriers make these kind of announcements, the rebranding starts immediately. In SQ's case, they are saying they will start in 2020. What does that say? It says to me that they haven't fully committed to the timing yet. It seems convenient that the rebranding will start taking place in 2020, when it will take a couple of years to refit their planes, maybe 3-4 years. And then ... HSR is almost upon us.