Specifically why he started to allow landless men to join

That is the so-called "Marian Reform," there's not really anything to be specific about. That's it, that's the entirety of Marius' legislation regarding the army, and even then it was surely not as revolutionary a decision as is often made out--the property qualification had been violated several times during and after the Punic Wars. The period following the Punic Wars was characterized by changes in land policy and military organization, to which Marius' sole contribution more or less was the final abolition of the property requirement. Most of the things that popular culture associates with the so-called "Marian Reforms" don't derive from Marius at all. For example, the discipline and in particular the reformed arms drill of the legions was instituted largely by Marius' colleague as Metellus' legate and long-time enemy, Rutilius Rufus by 105. Marius only further refined a reorganization that had already been largely completed, giving his men a little more training before combating the Teutones. The cohort had already existed during Scipio's Iberian campaign. The legislation that made it a normal expectation for consulars and praetorians to govern provinces and command provincial armies with promagisterial imperium (possibly the most important military legislation of all) is something that comes from Sulla, not Marius at all. For that matter, the practice of enlisting voluntarily for booty probably didn't kick into full swing until Sulla, whose army in his campaign against Mithridates was notorious for having plunder as its only motivation--Appian mentions that Sulla prevailed on his troops to march on Rome partly by pointing out to them that Marius might raise other soldiers to prosecute the war and that they would miss out on their plunder, for which Appian says the troops were eager. Marius' army was probably not made up predominantly of volunteers--the OCD notes that "Undoubtedly conscription along the normal lines still continued, but many volunteers probably chose to serve for sixteen years." Important military institutions like the auxilia did not exist under Marius--the auxilia owe their origin to the result of the Social War, which enrolled Italians into auxiliary cavalry squadrons, and to the practice during the later 1st Century, B.C. of using allied and tributary contingents alongside the legions. The auxilia that most people would think of when the term is used, the professional, permanent force that accompanied the legions in their frontier camps, didn't exist before Augustus.

In any case the details for Marius' abolition of the property requirement come mainly from Sallust and Plutarch. Plutarch gives no reasons, but Sallust says:

Ipse interea milites scribere, non more maiorum neque ex classibus, sed uti libido cuiusque erat, capite censos plerosque. Id factum alii inopia bonorum, alii per ambitionem consulis memorabant, quod ab eo genere celebratus auctusque erat

Meanwhile he himself drew up troops [scribere here might actually mean "to levy," as it often does--it's a bit unclear how Sallust imagines these troops are drawn up] not according to the old custom nor from the property classes but those willing, many from the lowest census classes. Some said this was done for want of good men, others for the canvass of the consul [the Latin ambitio is a candidate's canvass for office--the idea is that Marius may have abolished the property requirement to secure votes in the future or to repay his debt to those who had supported him. Of these two I think the latter is much more likely, since the majority of Marius' political career was spent repaying favors, particularly his attempts at land distribution, to those who had helped him], because by those kinds of people he had been celebrated and supported

So Sallust proposes a couple of possibilities, of which the first (manpower shortage) is usually taken as the traditional orthodox view. But in any case the property qualification was very much on its way out and was an antiquated tradition that had long lost relevance, even in practice

There is a lot of historical debate on this. I believe the common consensus is that this was a gradual reform that had been taking place and the Romans attributed it to Gaius Marius.

Letting men join the army below the property limit started as early as the 2nd punic war, if not earlier. The simple reason is that the romans needed to field more men to continue their wars. After the 2nd punic war and the conquest of Greece, the well-to-do Roman citizens wanted less time in the military and more time to devout to pleasure. So the property requirement gradually dropped lower and lower until the headcount could join the ranks as a way to improve their lot in life. Eventually this became fairly common practice, most likely during the consecutive consulships of Gaius Marius.