It's the 100MHz version being discontinued, not the model line. Our best guess about the reasons behind that decision are about price and features segmentation.
The base price is going to be €2.228 + VAT, or thereabouts, for the SDS2204X-HD. That is why you can only buy the 100MHz version if it is on stock, as Siglent can just swap the label on the front. The hardware is identical regardless of bandwidth.
With the release price of the 3000X-HD models, the previous 2000X-HD pricing made no sense. They had to slash the MSRP heavily. By making the 200MHz version as base model, they can get away with not lowering it as much.
So, right now, the effective price reduction of the 2000X-HD is €400, instead of €800 for the same hardware if they had kept selling the 2104X-HD.