I think one possible reason could be that the publisher has a number of journals under their purview, and perhaps the first journal you submitted to may not be as suitable for the paper which you submitted. Another journal under the same publisher may be more suited for the topics your paper is covering, so that may be a reason they asked you to resubmit to another of their journals.

Another simple reason is that your manuscript is actually "good enough" yet not outstanding enough for the primary journal to which it was submitted. When a publisher refers to its subsidiary journal, it will normally have a (high) publication fee, while guaranteeing fast processing and early publication. Meaning that as established publishers, they still want to publish your work as "second-rated" article if you are willing to make payment, and usually at exorbitant prices.

This is the dark side of the open access system. Publishers create new online journals as spin-offs from their existing ones to accommodate the ever-increasing research production. Even the giants in academic publishing such as Springer Nature has Scientific Report, Elsevier has Heliyon, and PLOS has several. Their publication fees are what can be appropriately described as astronomical.

I found a discussion regarding Heliyon which might interest you:

https://www.researchgate.net/post/Have_you_been_cheated_by_Elsevier_yet