You are right, as an aside Crassus was in effect a very wealthy real estate developer and military success was a way to higher office. He simply was in over his head in military matters.
The effectiveness of Parthian archery is not to be underestimated. I understand that they had a quiver with 70 arrows, and could fire quite a few each minute. The figure 15 per minute comes to mind, but my memory could be off on that. The battle of Crecy which was won by English longbowmen also comes to mind. Crassus may also have been tricked into believing faulty/destructive information about water, and travel conditions, in the desert in the summer.
Did some more Goggle at wikipedia, Battle of Carrhae. Found this quote regarding my opinion that Cassius may have been motivated by Caesar’s ambition to restore Roman honor, recapture the Roman eagles lost at Carrhae, as well as his dictatorial ambitions.
“Gaius Cassius Longinus, a quaestor under Crassus, led approximately 10,000 surviving soldiers from the battlefield back to Syria, where he governed as a proquaestor for two years, defending Syria from Orodes II’s further attacks. He received praise from Cicero for his victory. Cassius later played a key role in the conspiracy to assassinate Julius Caesar in 44 BC.
Parthian Horseman, §Legacy[edit]
The capture of the golden aquilae (legionary battle standards) by the Parthians was considered a grave moral defeat and evil omen for the Romans. At the time of his assassination, Caesar was planning a retaliatory war. The Parthians are said to have feared especially harsh retribution if Caesar won, because the surviving son of Crassus would be among the Roman forces.[37]
However, the fall of the Roman Republic intervened, and the beginning of imperial monarchy at Rome followed. Sulla’s first march on Rome in 88 BC had begun the collapse of the republican form of government, but the death of Crassus and the loss of his legions utterly reconfigured the balance of power at Rome.[38] An old theory ran that the death of Crassus, along with the death of Julia in 54, Pompey’s wife and Caesar’s daughter, may have severed the ties between Caesar and Pompey; the first Triumvirate no longer existed. As a result, civil war broke out. Caesar won, and the Republic quickly became an autocratic dictatorship. But several astute historians[who?] note the lapse of time between Crassus’ death and the outbreak of civil war. Gaius Stern has claimed that Crassus’ death nearly cut the links the First Triumvirate enjoyed with the blue-blooded aristocracy, leaving the entire state vulnerable to the friction that eventually turned into civil war.[39] Thus, an immediate effect of the battle may have been the elimination of certain private checks and balances (e.g. Crassus’ relationship to Metellus Pius Scipio) that formerly kept a lid on political tensions.[citation needed]”
I also read that Crassus’ son Publius may have persuaded the old man to go after the Parthians to further his own military ambitions. Well, it got him killed instead.