Why dipped traffic at low frequencies?

Question

Search Engine Optimization

The site was completely redesigned last year. Also, all content was completely transferred to the new site, but only the URLs where this content is available were not preserved.
The new site was successfully indexed by PS.
Content is currently published more and it is better than on the old site.
The site has been optimized: duplicates have been removed, CNC has been made, a sitemap is generated, a correct 404 error is generated, meta tags are written.
It has been 10 months since the launch of the new site.
But the difference is 40% in low-frequency traffic on the new site in relation to the old one, although the content is all in place.
Why is that?
And how to return low-frequency traffic?

Answer 1

Search engines rank specific pages of the site for queries, you removed these pages and gave the robots a 404 code - they could consider that the content had disappeared and start ranking other URLs again.
It was necessary to set up a 301 redirect immediately after the transfer. If 10 months have already passed, then there is probably no point in doing a 301 redirect, but you can try ... (especially if past pages have incoming links).

Answer 2

As I understand it, you transferred all the content, but did not put redirects (or did they?) in this case, search engines can consider the new site just a clone of the old one, respectively, give priority to the old one.
Another option is that the old one may have a trust, some kind of link mass, so search engines still like it more.

Answer 3

You forgot to redirect from old pages with content to new ones
from this you lost part of the link base, hence the drawdown