1. HK was a middleman, it used to connect the mainland China to the world. That worked for HK very well after 1949, but clearly the people living in the mainland do not want to see HK to keep having its cut for the flow of capitals/goods for a very reasonable reason - there are more and more professionals/businesses in mainland China that are eager/qualified/happy to take over what HK was offering.
2. HK failed to develop its industrial base. Let's compare it to Singapore, you see different high tech companies in that city nation, but what HK has developed in the last few decades? You can also compare it to the neighbouring Shenzhen, if Shenzhen can offer companies like Tencent/DJI, why HK can not do the same in a so called free environment? FTA is long available to HK, there are numerous HK based companies doing business in mainland and making good profits, just in the wrong sector - they are all in real estate. Rather than blindly blame Beijing, there are more meaningful reasons to look at.
3. HK's retail sector is no longer comparable to what you can expect from tier-1 Chinese cities (Shanghai, Beijing, Shenzhen). Mobile payment, online shopping, delivery, large and easy to use e-commerce sites, you name it. Sure you can easily argue that HK has a few department stores with pretty good services, but is that enough to support the future of a city with millions of residencies? The consequence is simple - Chinese from the mainland stopped going to HK for shopping, they start to feel its out of date payment systems, 1990s style department stores are getting less and less attractive.
4. HK's Infrastructure is no longer considered as good. I first visited HK 13 years ago when HK's metro was far better than anything you could possibly expect in Shanghai. Today there is high speed rail that can get me anywhere within 200km radius of Shanghai, Shanghai metro's scale is 2-3x times larger. How fast you can get yourself from HK's central to Guangzhou? I can get myself to a nice office building or hotel in Hangzhou from my home in Shanghai door to door in 90 minutes.
5. HK is losing its financial capital status fast. Chinese Yuan will be internationalised eventually, that is probably THE biggest financial event in the next 30 years, but how many people would possibly believe that having HK dollar in HK is a smart move? Let's remember that HKD is never allowed to freely float, its exchange rate is fixed to USD.
All this come to a real question here: is HK declining or just returning to where it suppose to be?