Why was the Marian reforms' inclusion of landless masses necessary?

As the territorial extent of the republic grew, the citizens who were eligible for service in the military were seeing ever increasing terms of service. The vast majority of the citizenry who served as infantry in the republican militia were landowning farmers who, as a result of the growing extent of the republic, were seeing deployment to provinces such as Hispania, Asia or Syria.

The system of levy established during the early republic which depended on these citizen farmers would deploy for a single campaigning season, coinciding with the planting and harvesting of crops. As a result of the extension of length of service in further afield provinces the citizen farmers would no longer be able to return to their farmsteads in order to plant and harvest their crops and so their land fell into a state of neglect.

The senators of Rome were not permitted to engage in any mercantile activities to generate income and so the acquisition of land was a priority (however it must be noted they were not the only rich individuals engaged in such practices). The neglected farmsteads of the citizens soldiers serving in the provinces were acquired by the rich who would employ slave labour to maximise the profitability of the estates and subsequently these events and circumstances created a number of issues, two of which were:

1. A severe lack of eligible citizen farmers for military service and;
2. Mass unemployment for citizens

Plutarch notes in the Life of Tiberius Gracchus Ch.8:

Of the territory which the Romans won in war from their neighbours, a part they sold, and a part they made common land, and assigned it for occupation to the poor and indigent among the citizens, on payment of a small rent into the public treasury.

He goes on in greater detail about the causes of the symptoms you mention in your question which continued well into the first century B.C. which the Gracchi and others attempted to rectify with Julius Caesar being another.

And when the rich began to offer larger rents and drove out the poor, a law was enacted forbidding the holding by one person of more than five hundred acres of land. For a short time this enactment gave a check to the rapacity of the rich, and was of assistance to the poor, who remained in their places on the land which they had rented and occupied the allotment which each had held from the outset. But later on the neighbouring rich men, by means of fictitious personages, transferred these rentals to themselves, and finally held most of the land openly in their own names. Then the poor, who had been ejected from their land, no longer showed themselves eager for military service, and neglected the bringing up of children...

And as you have said, Marius capitalised on this and reduced the previously required property qualifications for service from property worth 3500 sesterces and having to supply his own arms to effectively no necessary qualifications. So the class known as the capite censi or "head count", i.e. those who had no property to qualify for being assessed in the the census, were now eligible for service in the military.

With the growth in territory naturally came an increase in bordering other states. One of the most notorious and organised of Rome's later rivals was the Parthians whom they often squabbled with over buffer states and border disputes as well as the migrating hordes of Gauls and Germans and rebellious, resistant Hispanic tribes. So yes, Rome did face a different strategic situation which required a greater level of organisation due to the increasing distance from Italy they engaged in conflicts and governance in.