As you may be aware, the BBC iPlayer is available on a myriad of devices including television sets, streaming boxes, Blu-ray players, computers, mobile phones and more. Supporting and updating all these different versions is an expensive and time consuming task, particularly when you consider that some of the older devices may only have a few thousand users left. In any case, many people will have a more modern, alternative device - it is pretty hard these days to buy anything TV-related that doesn't have iPlayer on it.
Officially, the technical reason is that TVs and such from 5 years ago or so (or 2 years in the case of the Bush) use the MHEG version of iPlayer, also known as iPlayer V2, which the BBC will no longer support. But here's a coincidence: from September 1st, you need to have a TV licence to use iPlayer, whereas previously it was exempt. The suspicion is that newer versions of iPlayer include the technology for enforcing or at least monitoring usage, which can then be linked up with licence enforcement. Maybe the technology cannot be incorporated into the older versions, or at least not cost effectively.
If you are still using, say, a Sony TV from 2010 and are affected, what can you do? The simplest solution is to buy a modern streaming box, such as those from Now TV (£25) or Amazon (£35). These are likely to be supported for many years to come, plus will provide a whole host of additional capabilities.