What you are asking is a lot like "My Pentium 90 system still boots. Why doesn't Windows 10 run on it?"
It is not only the flash that is the problem, but also the RAM and the CPU power. People expect and need more from routers that are generally a decade old (or under-spec-ed by an ODM to reach some price point) than they did in the days when a 25 Mbps line was considered screaming fast.
With routers that support current wireless protocols that have reasonable flash, RAM, and CPU power available for US $20-40, it's hard to argue for spending development and testing time and effort on obsolete devices. Some argue that support for 4/32 devices should be dropped entirely and the focus should be only on devices that have sufficient resources to meet current needs.
Edit: There was quite a lengthy discussion on this in the beginning of 2017: