Not enough of them for one thing. Would've still required a second fleet of new power and all the associated costs and pitfalls of maintaining two fleets of electric locomotives, which would've only been slightly better than what they had before with two variations of AEM-7's and the Bombardiers.
And my impression was that Amtrak didn't have much flexibility before the 70 ACS-64's. Since they're not going to rebuild one locomotive at a time, what happens when say six AEM-7's are away for a major capital rebuild that turns them into essentially new power? And safety standards with crash-worthiness have advanced a lot as well since the AEM-7's were built.
I bet the ACS-64 is cheaper than trying to refurbish old locomotive bodies, recondition certain components, and essentially replace over 90% of the mechanical components with new technology. These Siemens locomotives, while apparently not perfect judging by threads such as over at Railroad.net with individuals that work on them talking about various issues, still have a lot to offer.
Proven technology, a single fleet of power and all the advantages that offers to Amtrak, enhanced safety for the crews operating them, and flexibility with Amtrak's existing locomotive fleet that was able to take up the slack as they slowly entered service and proved themselves. Not to mention that I assume there's some commonality with the Siemens Charger that looks set to be the major North American diesel passenger locomotive of the next few decades.
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