First, I wouldn't discount foreign support for the Nationalists. With the military having a roughly even split, the main manpower advantage for the Nationalists came from the Army of Africa. The Republicans controlled the naval forces between Africa and the Spanish mainland, so that crucial formation would have stayed out of the war if it weren't for foreign actions. This came in the form of a loan of transport aircraft from Germany to the Nationalists, who then conducted the first mass military airlift. The Army of Africa was ferried through the air to the mainland and rapidly became the deciding factor in the Nationalists' early successes.
Further, with the manpower on both sides being relatively even, the Army of Africa was not the only source of a numbers advantage for the Nationalists. The Italians--seeking to undermine the abilities of France and Britain to challenge Italian ambitions in the Mediterranean--poured a large number of troops into the Spanish Civil War on the side of the Nationalists. At its peak, this was a force of 50,000 (relatively) well-equipped soldiers. Their armor force could only be countered by Soviet tanks (which were in very short supply), and their aerial contingent was also potent. The Italian Navy also provided ships for the Nationalist blockade. While the German contingent was smaller (less than 10,000 at peak), their Condor Legion was decisive in the air (being stationed near the front of just about every Nationalist offensive), and they also provided needed training for Nationalist forces.
But if we do put foreign involvement to the side the Republicans still had several deficiencies during the war. Their offensives suffered due to a relative lack of machine guns, armor and antitank guns, and artillery. They were able to put up stubborn defensive efforts with mostly rifle-equipped troops, but offensives were most often limited to local gains. Once the Nationalists shifted forces to bring more artillery, armor, and automatic weapons to bear such offensives were pinched off and then the Nationalists could mount a counter-offensive.
The infighting amongst the Republicans (mentioned by another user) certainly didn't help their situation, but seeing that as causal for the Republican loss seems to be a bit of a stretch to me (it ignores that the Nationalists weren't a unified body and overplays the fighting that did occur between Republican factions). An incomplete list of other contributing factors to the Republican defeat would include:
Difficulties in purchasing arms abroad (the Soviet Union and Mexico being among the largest suppliers, with France, Britain, and the US refusing to sell them weapons)
A relative lack of organization at the beginning of the war, international distaste of violence against noncombatants (Particularly anticlerical violence. Also, it should be pointed out that the Nationalists avoided such condemnation of their own war crimes by censoring the press)
Internal conflict regarding priorities, with some members of the Republican faction choosing to pursue revolutionary political agendas as the top priority when hindsight shows us that focusing on military concerns might have been more prudent
World War II not starting as early as some on the Republican side hoped obviated the hope that France and the UK would intervene to stop Germany and Italy on the continent
There are more reasons, but I think I'm hitting the main ones. In the final analysis, the Nationalists had a manpower advantage and they used that to continually retain the initiative and grind down the Republicans. In the process, the Nationalists ignored a number of opportunities to win the war more rapidly. Franco chose a plodding campaign which allowed him to "cleanse" conquered areas of Spain of leftist influence. Franco also chose to launch immediate counter-offensives whenever the Republicans were able to have a successful offensive, and he did so with the sole goal of winning back the lost territory. This showed a dedication to this policy of cleansing the left from Spain, and displayed a lack of desire to win the war quickly. It would have been much more efficient to attack on a more strategic front while the Republicans were stretched thin, but Franco's desire was for a decisive ideological victory rather than a rapid military victory. At no point were the Republicans able to have anything other than local success in seizing the initiative. They remained on the strategic defensive for most of the war, and eventually lost a battle of attrition.
I hope that answers your question. Followup questions from OP and others are always encouraged.