I'm far from being a military historian, but the short answer to this is, by my understanding, that France was the stronger power at this time.

France had a proven military strength backed by competent leadership and an ability to mobilise as well as a wealth of colonies to draw military and economic support from (notably holding a number of colonies in Africa, a key area for German expansion). By comparison Russia was a seemingly weaker and poorer nation, their armed forces were poorly equipped and disparate and morale was presumably low following their defeat in the Russo-Japanese War in 1905. Likewise, the Russian populace was spread over a vast area and predominantly rural, making conscription a much greater task.

On that basis, the Schlieffen Plan sought to exploit a predicted small window of comparative weakness and break their perceived encirclement. The Triple Entente was largely untested at the time (aside from smaller diplomatic tests in North Africa (Agadir, 1911)) and there was an assumption that Britain would be reluctant to commit to a war in Europe on behalf of either France or Russia, and the treaties they had made didn't establish any obligation for Britain to join a war with the Entente powers (this in contrast to the Triple Alliance). And even if Britain did join the conflict, it would take them time to mobilise and move their small army across the channel - by which time, it would be too late. This meant that France effectively would stand alone as the power with the initial military capacity to compete with Germany.

As such, the plan was to move quickly through Holland and Belgium and into France through a less defended point (decades of French/German conflict ensured that their shared border was well defended and seen as an obvious point of attack), advancing straight into Paris and forcing the French surrender within a matter of weeks. This would remove the main continental Entente power quickly and without engaging with the vast majority of her forces - the idea being that Germany could then focus on combating the weaker (if more numerous) Russian forces on a single front, ideally before they had managed to fully mobilise. Had they decided to attack Russia first, the German forces would have faced a gargantuan logistical task to fight across a large area to take the Russian capital and leave what was perceived to be a much more powerful and efficient military preparing at their backs

Ultimately the German offensive was stalled by staunch defence in Belgium, supported to Germany's surprise by the British Expeditionary Force. At the time Britain operated a laissez-faire approach to a lot of global politics and aimed to maintain a policy of being at least as powerful as the next two European navies - essentially creating a system where they felt safe against any European maneuver, and didn't need to be directly involved. What Germany had not expected was that Britain would join a war to defend the Treaty of 1839 which dedicated Belgium as "... an independent and perpetually neutral state..." - this was disparaged in 1914 by Chancellor Hollweg as "a scrap of paper"

The days/weeks by which this slowed the German advance allowed time for French forces to prepare more effectively and for British and Empire forces to continue to bolster their defence. Aside from slowing the assault, this forced Germany to commit greater numbers from the Eastern Front to the Western Front. In addition, Russian will to fight proved much greater than expected and mobilisation took a lot less time than anticipated, resulting in a larger and stronger Russian force than expected in a much shorter time. There was a small window of opportunity, which was ultimately closed a lot faster and more forcefully than the German command had anticipated.