General Joffre had no men to spare to be bottled up in the city.

He determined that his armies should be kept free on the field.
You may ask anywhere in France, Belgium, or England why the French did not come to the relief of Belgium, why Paris was undefended, and what saved it after Von Kluck had led seven armies of 1,000,000 men down to its very gates, and you will get no satisfactory answer.
But when you have studied the situation and the record, you will see that no simple answer can be readily given.

A brief one would be: French mobilization plans were imperfect, and, therefore, Belgium could not be defended by the French.

But motor-busses did what the railroads were unprepared to do, and finally saved Paris and France.
The French had been warned many months publicly and privately that their mobilization plans would be found faulty in case of sudden hostilities.

The railways moved perishable goods at the rate of thirty miles a day while German and Austrian railways bore military trains at the rate of thirty miles an hour.
So ill prepared were the French in their mobilization plans that they actually summoned to arms the men who were to man the railways, and the railways themselves were deficient in rolling-stock to move the troops.
The citizens responded promptly enough, but France had no bureaucracy or military plans to match those of Germany, and, as throughout French history, the leaders of the people failed at the crucial moment.