Hi Frank,
The answer is the usually crap software of cheap WiFi routers aimed at consumers. There is no need to write decent software, as they only have to appear to work when first turned on, so they don’t get returned to the shop. Over time they fill their internal tables, have memory leaks and other failures due to poorly written and tested code. The easy way to work around this is simply to power cycle the router but that sucks more and more over time.
Good quality routers will run for a long time without slowing down. While the hardware is sightly better quality, the real difference is properly written and tested software.
In addition, it is very easy to flash Asus routers with alternative software, which might be useful if an update causes problems at some point.
https://www.asuswrt-merlin.net/
My previous router was also an Asus, which worked reliably up until the short circuit. I’m familiar with the user interface, which allows to make almost any setting.
Another reason, because I am dependent on a reliable network for professional reasons, I have an additional DSL line that is handled via a Fritzbox 7590. Quite okay the FB, but not nearly as good as the Asus. The WiFi range of this one is much better, the signal is stronger, the throughput much higher and the performance of the Asus does not drop even when the whole family is gathered in the big house for a week over the Christmas holidays: In addition to smart home devices, several TVs streaming at the same time, people working on laptops, phones, tablets, running Tonieboxes, and me while cooking listening to roon in the kitchen. The Fritzbox couldn’t get that without performance drops.
But that’s just me. 
Others have different preferences and I encourage you to read through this thread:
Hoping I could be of some help, good luck!