Because, as I previously said, the logic of a conflict is to find allies in order to improve the balance of powers in one's favour. The neighbors are not going to stay watching if they have any interest in intervening.
Britain had no alliance, but this could change. Especially given which country was France's nieghbor in the east. Besides, what would the french economy have felt if it was blockaded in an age when trade became much more important than it had previously been ?
Russia at that tiime did not have the military means to follow France in a conflict against Britain. It had no interest in doing so.
There were quite many heads of State who would have either sided with Grandma Victoria or who would not have stood against Grandma Victoria.
It might be interesting to note that, as late as 1900, provisions of the Franco-Russian agreement included provisions stating that in the event of an Anglo-Russian war, France would move 100,000 men to the Channel coast; in the event of an Anglo-French war, Russia would move troops to the Indian border (along railways constructed with French money).
The Definition of Understanding signed by France and Russia in 1891 described one reason for their treaty as "the probable adhesion" of the British to the political aims of the Triple Alliance.
We must remember that there was enough evidence of Anglo-German rapprochement in the 1890's to give the Franco-Russians cause for concern, and also that Russian ambitions in the Far East were at that moment causing tension by conflicting with Britain's China Policy. Britain and Russia had not yet signed the Convention calming the tensions of the Great Game; as George Curzon's words show (he was made Viceroy of India in 1899), the British were still very concerned about Russian intentions in Iran and near India.
So while it is highly unlikely that Fashoda would lead to war, i think there is a real possibility that Russia might be drawn in if one somehow broke out. There are not only treaty terms that make it more likely, but also an environment of deep suspicions (on both sides) would contribute.