Of course, everything depends on the buyer's budget, but for providers it becomes more and more expensive to maintain old CPUs. At the same time, it is simply impossible to make entry-level configurations with 4-8 cores even cheaper - their maintenance for many DCs is already almost at the cost price, while providers are often forced to undercut prices for relatively new configurations in pursuit of customers.
As a result, old Xeon processors, especially those that come paired with DDR3 are simply not relevant either for the client - because for this price it is almost always possible to find newer ones, or for providers who simply do not find it profitable to keep a fleet of configurations with low demand.
Although I would like to note that outstanding CPUs with high clock frequency and relatively successful architecture are still interesting and useful. Intel Xeon E3 v6 and in general Xeon Scalable CPUs on Skylake and Cascade Lake architecture still look good for budget configurations like storage servers, web servers, some test environments etc.