1. I really like both series. In fact I started watching The Apothecary Diaries because I missed Raven of the Inner Palace... So in no way am I intending to hate on either series with this post...
2. While I'm currently reading the light novels of The Apothecary Diaries—and am on volume 8 presently—I haven't read any of the light novels of Raven Of The Inner Palace. Therefore I can only base my arguments off of the anime for the latter series, though I definitely plan on reading the novels at some point... That being said, I have included a spoiler for the finale of Raven Of The Inner Palace's light novel... However I have clearly marked it, so you can skip over it if need be...
3. From here on out I will be referring to both series by their respective Japanese titles as it's grammatically easier for me to use them in sentences.

While a multitude of factors can contribute towards a disparity in popularity between two series, I believe the main factor at play in this case is the differing demographics of both series. Although Kusuriya no Hitorigoto is somewhat ambiguous in terms of demographic—considering the novel was initially aimed at women—it is largely considered to fall into the seinen bracket because its manga adaptation is serialized in a seinen magazine. Kōkyū no Karasu, on the other hand, is exclusively categorised under the josei demographic.

Throughout the history of the animanga industry, women have never been prioritised and accommodated for in the same manner that male audiences have. This is evidenced by the limited adaptations, merchandise and promotion shoujo and josei series receive relative to their shounen and seinen counterparts due to a perceived lesser profitability. Kusuriya no Hitorigoto has almost certainly benefited from being rebranded as a seinen. This change in demographic has undoubtedly resulted in the series receiving more investment resulting in better promotion and more adaptations—ultimately allowing it to reach a larger audience than Kōkyū no Karasu. The difference in production quality of both series’ anime can also be attributed to how adaptations of shounen and seinen series oftentimes have more polished adaptations—due to increased investment—which helps further bolster their popularity.

Although not everyone is the same, most shoujo and josei fans enjoy a wide range of genres and demographics, whereas the vast majority of shounen and seinen fans only consume shounen and seinen animanga. The "rebranding" of Kusuriya no Hitorigoto into a seinen series has allowed it to resonate with a larger audience compared to Kōkyū no Karasu—thus resulting in the former series achieving higher sales and greater popularity.

Many of the executives in the animanga production industry mostly are men, who not only see greater economic potential in own demographics, but also have misguided assumptions about the demands and interests of the female audience. Furthermore production companies are more hesitant to invest in shoujo and josei adaptations—so when they do invest in these demographics, they tend to stick to what appears to be a safer option. Consequently shoujo and josei series with romance-focused narratives are pushed to the forefront as it is assumed that is what female audiences want. Shoujo and josei series that deviate from this pattern are often assumed to have a limited appeal and thus aren’t given as much investment and promotion. Whereas the shounen and seinen demographics are littered with series that prioritise genres other than romance. As a result, even though Kusuriya no Hitorigoto and Kōkyū no Karasu both emphasize topics and narratives beyond their romantic subplots—the former series likely encountered fewer obstacles in obtaining investment opportunities compared to the latter due to its shift in demographic. [Potential spoilers in the following paragraph!] Furthermore, the conclusion of the Kōkyū no Karasu’s light novel—which chose to solidify the relationship between its female and male leads as platonic rather than romantic—likely further limited the series’ potential for further investment opportunities due the prioritization of romance-driven narratives in series aimed at a female audience.

-> Kusuriya no Hitorigoto's logic and science-based approach to solving mysteries likely appeals to a wider audience than Kōkyū no Karasu's mystical and fantastical elements. -> Kōkyū no Karasu expresses its character development and themes more subtly than Kusuriya no Hitorigoto does, potentially making it less appealing to broader audiences. -> The highly stylized visuals in Kōkyū no Karasu’s anime have resulted in the series having a more niche appeal than Kusuriya no Hitorigoto’s more straightforward presentation. -> Kōkyū no Karasu has always had to bear the brunt of comparison against Kusuriya no Hitorigoto, considering the former began publication in 2018 when the latter had already captured a substantial portion of the audience’s interest within the same niche. -> One of the primary critiques levelled at Kōkyū no Karasu’s anime is that it feels rushed and thus lacks depth and memorability, especially when juxtaposed with Kusuriya no Hitorigoto. However unlike the latter series, which had a 24-episode first season to meticulously develop its plot points and characters, Kōkyū no Karasu only had 13 episodes.