You are thinking of Mazda's KL series v-6 engines. It was made in several different versions from 1.8 liter to 2.5 liter. They won a bunch of awards when they were introduced and actually it is a very good engine.
I don't know exactly why Mazda discontinued it but I suspect it has to do with the development of the harmonic balance shaft and variable valve timing technologies. The first allowed a four cylinder engine to run nearly as smoothly as a 6 cylinder engine and the second allowed the same four cylinder to deliver a higher specific output.
If you stop and think for a minute, you will see that it is easier and cheaper to add these technologies to a four cylinder engine and develop a car around this "modern" 4 cylinder than a comparable 6 cylinder. The 4 cylinder also allows for greater flexibility in installation since it takes up little space.
The other thing that might contribute to the demise of the KL series is that it was designed to be installed leaned over to the side. This made packaging in a small front wheel drive car easier. But it also meant that all the oil passages and such like are arranged to support this angled installation. The only things I have found talking about trying to mount this engine upright have discussed poor oil distribution. This pretty much precluded the installation in an upright position and made a longitudinal mounting difficult.
I don't know if any of this answers your questions or just raises new ones. Remember this is my speculation so take it for what it is worth. By the way, one of my cars is a 93 MX-6 V-6 which is one of the cars that used the
2.5 liter version of the KL. I bought it new in May of 92. It is my version of a sporty utility vehicle and I still love the thing. It is not as much fun and frisky as my Miata but then the Miata doesn't carry four people or bicycles inside.
links you might like to read:
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For what it's worth.