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Career porn artist here. The reason is Lana is already a popular design so "Lana, but a milf" is also a popular design. There's a reason Lana's mom (and even younger sisters) beats out even legacy characters like Brock's mom and even Ash's mom in spite of their seniority, it's because we basically just got an aged up version of one of SM's already most popular girls, this is also why she's very very commonly also drawn with Lana in her art, which makes the appeal obvious to anyone with half a brain.
A more generic milf design that has no real attachment to any other popular character isn't gonna do anything, Pokemon artists already have plenty of options if they want fat tiddy milfs like that ice mom lady from the worst games, Katy, Wicke, Lorelei if you're a boomer. There's also the option to sex up non milf characters as we see with May and Hilda....basically there's a long line and you need a reason to stand out (as Lana's mom does) to get people to draw your design.
tl;dr Lana's mom has a unique appeal based off a hit design, it takes more than just being a fat tiddy milf to garner attention from nsfw artists.
