I wouldn't say it was a matter of choice. Nadine's site has had guest models for ages, while Milena's site was always Milena only. So when Milena decided to wind down, it was quite logical to make her another "guest model" on Nadine's site, instead of moving all the guest models from Nadine's site to Milena's site.
However I do agree that there's not enough Nadine updates. Only one new photo set every 3-4 months since last summer. But it's good to see both Nadine and Milena still modeling, after 15-20 years from the start.