And Michelin must have a phenomenal lobbying team if they managed this: https://en.wikipedia.org/wiki/Rubber-tyred_metro#List_of_sys... (In particular, "didn't catch on outside France" is, well, just plain wrong.)
The list once you exclude that basically consists of
* France, which wanted a niche technology to export and maintain supply domination over
* China, Japan, and Korea, trying to replicate it
* Quebec, which often looks to France for planning inspiration
* Mexico and Chile, which were recipients of French development assistance that included using French suppliers
Even in the countries where they exist, rubber tired metros are niche.
You can't retrofit rubber wheels easily so of course compared to all metros as a new technology it will be rare
Retrofitting actually has some advantages for old lines with short station distances, tight curves and steep gradients, whereas new lines can be planned with the limitations of rail technology in mind.
Rubber tired metros are also a niche in new metros.
What a silly assumption. They are not.
Replicate the industrial strategy, not the technology.
Railways are also part of this strategy. China, Japan and Korea all export railway products from high speed rail to subways to monorails, and so in this context rubber tired metros are another niche technology to get an edge in. Note that this strategy doesn't always work; no one outside of these countries is very interested in maglev trains, for example.
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The other part of this is development finance. Countries that do not have know-how with big infrastructure projects often look to international institutions to build projects and secure financing. And part of the deal is that if you take French money you must spend it on French suppliers, and this is true for basically any country doing this kind of lending, so basically the lender is getting both interest payments and spending towards its economy. The Chinese and Japanese compete a lot for railway projects in particular; Japan built the Taiwanese HSR and the Hanoi and Ho Chi Minh City metros, China is currently building high speed rail in Thailand and Laos, etc.
And generally speaking 5 % wouldn't be considered very steep at all by modern standards. Gradients up to 4 or even 5 % weren't uncommon even on older rapid transit systems dating from a century ago, and for modern multiple units I'd regard 4 to 6 % as about par for the course.
And in hilly terrain, a number of systems definitively go beyond that into the 8 – 10 % range, including some new built systems (e.g. the light rail systems in Stuttgart, Tenerife and Jerusalem).