DIAS - why to use one?

That is actually quite an interesting question and I would hope that some other DIAS users would provide their insights as well (we are obviously biased, as you noticed)
First, let me mention, that Sentinel Hub is integrated and available on Mundi, Onda and CreoDIAS (in no specific order) and is being integrated to WEkEO. Sentinel Hub functions available on each of these are not yet fully aligned with our AWS-based service, but we are working on it and it should be finished shortly. Some more on this here:

Becoming a Data Driver in the Cloud for Earth Observation

Sentinel Hub’s Roadmap for 2019

Reading time: 8 min read

Generally though DIAS as its core is object storage filled with Copernicus data, catalogue service and VMs to rent. One using only DIAS, will still have to do all the data processing steps, which will require effort as well as ICT costs. This part can mostly be replaced with Sentinel Hub API.

Some DIAS-es are more operational than others and you might still find some issues on a few. Their data catalogue is also not yet fully populated. They also have different data offers, e.g. not all have Sentinel-2 L2A data, etc.

At some point there will probably be some special tools on DIAS-es, which are not available on other clouds, but for the moment I think the major differentiator is the data. E.g. you will not find Sentinel-3, Sentinel-5P data on AWS or Google…