My guesses:
-Maybe more Duo cards were produced than Pro cards.
-I believe the Pro card also works in the Duo, that could make it "better" than the Duo card.
-Perceived value, since it has additional RAM. Also, it's called Pro!
Any idea if they were the same price when new? If the Pro was more expensive, that could be a factor too.
For what it's worth, the price on both cards doesn't seem to have gone up much since I got my first Duo back in 2008. I paid $25 for a Duo card at the time, and I remember Pro cards being around $60.
From what it looks like the Arcade card pro will not provide extra benefits to a Duo system. Well, at least I don't think it will.
It has the same amount of extra ram as the Duo card except it has the extra 192kB that the Duo's already have. No idea if using the Arcade card pro overrides the ram bank in the Duo console itself thereby making the data go to the Arcade Card pro or if the console still uses it's own ram bank. I wonder if there is a speed difference in this scenario, faster or slower?
This is speculation on my part as I generally have no idea how the system communicates with other parts of itself but it intrigued me to think about that extra video ram on the Arcade Card Pro and if it could be used or if it did get used when in a Duo system.
From wikipedia
Arcade Card Pro - 17.5 Mbit RAM (2240 kB as 2 MB+192 kB) – RAM upgrade. This greatly expands the RAM available for the CD-ROM unit to 2240 kB. The BIOS revision was unchanged from v3.00. Games developed for this System Card bore the title ‘Arcade Card CD’, and could not be played using an older System Card. The Arcade Card Pro includes the extra 192 kB needed by the non Duo CD system. The 2 MB of RAM is accessed through ports or units of single 8 kB banks and is intended for graphics data storage rather than program code; its flexible addressing system allows for rapid transfer of data to VRAM.
Arcade Card Duo – 16 Mbit RAM (2048 kB) – RAM upgrade. This greatly expands the RAM available for the PC-Engine Duo system to 2048 kB. The BIOS revision was unchanged from v3.00. Games developed for this System Card bore the title ‘Arcade Card CD’, and could not be played using an older System Card. This will only work on the Duo systems, as it does not include the extra memory built into the Duo system.