There's a lot that could be discused here on why Japan chose the time it did for its invasion of the European colonial holdings. The question of timing involves the lack of progress in the Second Sino-Japanese War, the souring of relations with the United States (and attendant financial and economic embargo), the changing balance of power in the north vis-a-vis the Soviet Union, Japan's relationship with Germany, and the internal politics of Japan at the time. You could easily go on for hours discussing all of these and why they meant Japan did not consider military invasion of the European colonial holdings earlier.

However, what was probably the most important factors in deciding the time of the Japanese attack was the combination of: 1) Germany's military victories in Europe; 2) the "time crunch" created by the US oil and scrap embargo.

As to the former, Germany's victories in Europe had seen the homelands of major European colonial powers (the Frenc and Dutch) occupied, while the British--still the strongest of the European powers in Asia--had their attention fixed on Europe and the German threat on the continent. Furthermore defeat of France meant the Royal Navy had to contend with the Italian Regina Marina for control of the Meditteranean, on top of escorting convoys across the Atlantic, and keeping the German Kriegsmarine bottled up. This presented a near unique opportunity for the Japanese, as there would be no grand movement of the Royal Navy to Singapore to oppose them, and the collapse of their homelands gave the Japanes a massive leg up in negotiations. German victories in Europe seemingly presented a unique opportunity for Japan that it would have to take advantage of.

As to the second, the Japanese occupation of French Indochina prompted the United States to cut off exports of oil and scrap iron, as well as freezing Japanese assets in the United States. This economic pressure was a serious threat to the Japanese war machine, which was running itself into the ground dealing with the Japanese quagmire. In many ways, Japanese leadership saw themselves in a "use it or lose it" situation, wherein they had a short period of time where they could employ Japanese military might to try and achieve a favorable negotiating postion, before increasing American naval power combined with a decline in Japanese military capabilities due to the embargo meant that Japan would be unable to wage war at all.

Thus, the move into South East Asia--while a long term ambition--was hardly part of a well crafted strategy. Rather, it was a reaction to the US embargo, and the need to secure the resources by which Japan could then "finish" the war in China, while simultaneously dealing with the United States before the US military advantage became completely insurmountable instead of only nearly insurmountable. The situation was simply not ripe for the scale of escalation a full scale invasion of the Dutch East Indies before the Geman victories in Europe, and the pressure created within the Japanese political and military decision making bodies by the US embargo.

Hope this helps to answer your question, and please feel free to ask any follow ups.