Buenos Aires was still much more populated than the others, and it's understandable why the unitarians thought it would work.
However, unitarism means the local
caudillos - that is, the province's large landowners
- would only get political power if authorized by an unitarian president, and that's something they would never agree to.
So, a unitarian victory would require a long term military occupation and would lack local support.
Mitre's victory at Pavon can be thought as an unitarian victory and, in many ways, so can Rosas long government/dictatorship. And both Rosas and Mitre sent military forces to the other provinces to ensure their rule. But, at the end of the day, they always had to allow the provinces to choose their own governors, even if they retorted to military force to ensure the chosen governor was one of their liking. Hence, the OTL result: Federalism, but with a very strong central government. I think the economy and demographics prevent a completely unitarian victory.
Is unitary Arentinans similar to unitary Mexicans ie they tend to fight for God, Army, and King aka Conservatives or Centralist
Generally, the unitarians tended to be regarded as europhiles, seculars and liberals; while the federals tended to be protectionists and pro-catholicism. However, at the end of the day, it was a matter of money and business, so the old Grouch Marx phrase 'this are my principles, if you don't like them, I have others' apply