In The Super Famicom #12 1991 there's an interview with Konami representative Sachiyo Sagoi about Super Castlevania IV. Apparently, "Akumajou Dracula" was chosen as the title for the game in Japan because it signified a "back to basics" approach for the series.
Nothing is said about why Konami decided on this approach. However, it's very likely it had to with CV2 and CV3 (which both tried to add their own unique elements to the series) underperforming sales-wise. According to VGChartz, the original Castlevania sold 1.23 million units in total. Simon's Quest sold 0.93 million and CV3 only 0.85 million. VGChartz is often considered to be unreliable, but there's also
this comment by one of Akamatsu's colleagues which attests he got demoted because his Castlevania sequels didn't sell well.
I'm guessing this back to basics" approach also influenced the decision to recycle the plot and main character of the first Castlevania, which would answer the question of: "Why is Super Castlevania IV a "remake" instead a sequel?". Though in that case, it would be more accurate to describe the game as a "clean slate" for the series rather than a remake.