There's a bunch of reasons, here's a few I've heard or thought of myself:
* DDR2 is generally 1.8V. There's probably already a need for 1.8V logic within this SOC, so the power rail is "free." DDR3, DDR3L, and all the variants of DDR4 use a lower voltage. No one wants another power supply rail just to power some internal-to-the-SOC memory if they don't have to.
* DDR2 is easily available in single die x16 (data bus width) 64MB (512Mb) sizes. DDR3 may be available in this size, but DDR4 is almost certainly not available or would require buying dies which don't sell a lot so becomes risky.
* DDR2 in single-die configurations is super simple to hook up.
* DDR2 in this size die isn't going away any time soon and may likely be the cheapest way to get a smaller amount of RAM.
* SIP vendors often can negotiate very good and stable pricing for RAM, which is a big benefit to the PCB design integrator as they then don't have as crazy of price fluctuations which are common in the RAM market. This makes the SIP more attractive to some customers (the PCB designer, not the end user).
* DDR2 can likely keep this speed of CPU fed with data fast enough that the RAM won't be the bottleneck for many workloads.