My understanding is that they bought the Budd built dome car because they could get it at extremely short notice. Following the Talgo mess in the Port Defiance accident there has been a crippling shortage of passenger rolling stock in North America. The wait time for new trains from the likes if Siemens is literal years.
The project was pushed forward by the President of Mexico and absolutely every stop was pulled out to get the system open in time for "AMLO" to have a triumphant ride over the line before the election in 2024.
The area is extremely impoverished and has terrible transport links, it's not really a particularly touristy area.
At the time of the Port Defiance accident, the Siemens Venture was likely only under very early consideration or not under consideration at all by Amtrak. The initial replacement passenger coaching stock for all these state corridors was supposed to be a fleet of Nippon Sharyo built next-generation editions of the Surfliner cars. 2 body shells were built and they both failed crash safety tests, leading for the program to be canceled and Siemens brought on in 2019 instead.
Outside of electric locos for the Northeast Corridor and the Brightline equipment, Siemens products provided to Amtrak and other US customers have not been doing great. Midwest venture coaches have been struggling to stay in service and chargers of both the SC-44 variety and the long-distance ALC-42s are repeatedly having failures (to the point Amtrak is now returning B32-8WHs, mostly reserved for yard switching, to revenue passenger service on Chicago-based intercity trains during the winter).
Equipment is stretched to its limit outside of the NEC due to minimum axle counts.
The Budd dome car was something they could get at short notice and likely for low cost - as Railexco, who owned the dome, the amfleets, and a pair of F59PHs now bound for Mexico signed a fixed rolling stock provider agreement.