Being that the Spanish and other Europeans would have arrived on the Caribbean side much earlier and would have had a much easier time operating on that side, why is most of the development on the Pacific side? The Caribbean side of those countries seem to be dominated by large nature preserves. Is it simple geography (rough terrain / poor ports / etc?) Did the Panama Canal have any effect on this?
How are you comparing the relative levels of development of the coasts?
The simple reason is that since the earliest days of colonization in the mid-16th century, there was a very important flow of precious metals from the very lucrative silver mines of Peru, and secondly there was a major trade across the Pacific with the Spanish colony of Manilla in the Philippines which had access to very lucrative spices and silks and other goods from China and India.
Because of this, the cities of Lima in Peru and Panama on the Pacific side of the isthmus served as vital collection points for the vast amounts of silver being produced. The majority of it was supposed to be shipped to Spain and this was done annually or semi-anually for over two centuries on the specially selected Spanish treasure fleets or convoys. After the silver was brought to Lima by mule train from the mines further south in PotosÃ, it would then be shipped to Panama and transported the 20 miles across the isthmus by more mule trains where it would be loaded onto the awaiting treasure fleet at Porto Belo. The English privateer Sir Francis Drake famously hid in the jungle and ambushed one of these mule trains in 1573 and stole lots of treasure, but for the most part this was the safest and by far the fastest way to transport these goods to where they could then be shipped to Spain. The English buccaneer Henry Morgan also marched across the Isthmus of Panama from the Caribbean and burned down the city of Panama in 1671, but it was rebuilt a few years later and quickly became even larger and more important.
Meanwhile, further north in Acapulca in Mexico, from the late-16th century onward it received annual shipment's of spices and silks and other precious commodities across the Pacific from Spain's colony of Manilla. These annual ships were referred to as Manilla-Acapulca galleons or just Manilla galleons and they sailed nearly every year from 1565 to 1815. After arriving at Acapulca, the precious commodities were then transported across the Isthmus of Tehuantepec (the narrowest point of Mexico) by mule trains and loaded onto the awaiting Spanish treasure fleet at Veracruz. There were several instances in which these galleons were captured by English, French and Dutch privateers and navy such as Thomas Cavendish in 1587 and George Anson in 1743 when they made incursions into the Pacific to raid the wealthy Pacific coast, but for the most part the galleons were insulated from most of the constant dangers they would have encountered on the Atlantic or Caribbean side.
However, all this said, there are also many large cities on the western side of the former Spanish colonies. There is Cartegena in Columbia, Havana in Cuba and many other towns and cities that have all been very important throughout Spanish colonial history and continue to be today to some extent. The reason there are also large cities on the Pacific side goes back to the mid-16th century and I don't know much about what effect the Panama Canal had on them when it was opened in 1914, but that was long after Spain's empire had vanished and the silver mines had dried up.
Sources:
The Manilla-Acapulca Galleon: The treasure ships of the Pacific by Shirley Fish
The Sack of Panama: Captain Morgan and the battle for the Caribbean by Peter Earle: this contains a good chapter on the silver mines of Potosà and the Spanish treasure fleet and how Lima and Panama were vital to this