Two main reasons with the first being that the Spanish Army of Flanders under the Duke of Parma was rather busy at this period - dealing with the Dutch Revolt, which had been going on for about twenty years at this point, and would continue, in various stages, for another sixty. Elizabeth I of England was supporting this Dutch effort, sponsoring troops and privateers to stymie Spanish efforts as well which was another reason for the planned invasion of England.

The Spanish government felt that Parma’s army might possibly be spared for a short time – a few days to transport them over the channel followed by small matter of weeks campaigning in England, before returning to the Netherlands – particularly following his reconquest of much of the territory from the rebels in the preceding years. The Spanish did not believe it would be wise to remove the Army of Flanders from the Netherlands for an extended period however, as there would be too much of a risk of a Dutch resurgence, not to mention opportunistic attacks from France, the Germanies, or other nations hostile to Spanish Habsburg imperialism.

The second reason was that attempting to move a large army from the Netherlands to Spain was a far more expensive, time consuming, and ruinous endeavour than having the navy move to them. Fundamentally, there was no proper logistical arm for the military of the period, with troops instead drawing their supplies from the local area. Furthermore, there was no direct land route from the Netherlands to Spain. Certainly, the army would not have been allowed to march through France. Even a marching army was a horde of locusts moving through at area, stripping the locality of much food and other supplies.

Phillip II of Spain was the ruler of Lombardy and Franche-Comte, so the traditional route of reinforcement to the Army of Flanders was through a range of military corridors through these territories and the linking foreign held-lands of the Duchy of Savoy-Piedmont, various Swiss cantons, parts of the Germanies and the Republic of Genoa on the Mediterranean coast. The movement of reinforcements from Habsburg territories in Italy, Naples, and Spain via the Mediterranean, required much planning and diplomacy, with the route planned, assurances granted that damage would be limited, and enough preparatory work to ensure that local civilians would establish markets en-route from which the marching men could purchase their supplies, and thus that these men with given enough ready cash meet their demands as the small forces marched through. The huge numbers of men, horses, and artillery of the Army of Flanders undertaking this route would have overwhelmed the local ability to keep them supplied. A marching tercio marching up through this corridor to the Netherlands would be one thing, after many weeks of preparation, marching between 8,000-10,000 men, as well as the associated multitude of ‘mouths’ which accompanied them and who depended on the army for survival (soldiers’ families, sutlers, merchants, carters, etc) would have been something different

So therefore, moving the army to Spain would have required one of two options.

1. Attempting to assemble enough wagons, carts, oxen, horses, and food (which would have had its own costs and issues for the cash-strapped Phillip II) to move the men. My own study for military logistics of the period suggests a unit of 1,000 men (Spanish tercios were supposedly 3,000 at full), eating the ideal military ration of 2lbs of bread and 1lb of cheese a day, would require an entire wagon load of food every two days. And this on the largest, best wagons of the time which were not common or plentiful to hand. Thus, an army would be requiring a constant supply and resupply of food along unmetalled roads, in winter and spring marching (the invasion was set for summer) with enough wagons and horses to allow some to be with the army, and some away being refilled, and the journey time in between. An organisational nightmare, impossible to achieve in the period.

2. Marching your army south through your own territory and the territory of essential allies and watch as your men stripped the land bare for thirty miles either side of the line of march (so an area 60 miles wide), with all the attendant destruction, anger, costs, and banditry which would accompany it.

Finally, the lengths of distance and time for a marching army would decimate the force as the journey would expose them to disease, hunger, desertion, and other hazards. As armies of the period considered themselves lucky to be operating at about 60-70% of full strength, it would have been a much smaller force which eventually arrived in Spain, and you would still have needed the Armada-sized transport fleet to move them there across the Mediterranean, from their embarkation points in northern Italy. All in all, not a great idea.

I am happy to answer any follow up questions, but if you are very much into this period and seek to know more of the Army of Flanders, I can recommend no better book than Geoffrey Parker’s seminal work: The Army of Flanders and the Spanish Road 1567-1659 (Cambridge, 2004)