I still see quite often that RJ45 DB9 console cable is attached with some devices, when I do not even have a computer with old serial port. So I need to either get the USB – DB9 adapter or USB – RJ45/Console cable separately. Is it just that it takes time to move away from the old serial or I am missing something?
Take a counter-example. We have some equipment that has a mini-USB serial port (or rather, a RS-232 signalling protocol carried over a Mini-USB socket/plug; thus not really USB per-se.
This is a big pain in the proverbial, as trying to find a mini-USB adaptor was quite challenging (particularly on the local market). We ended up having to order the particular adaptor from the vendor.
If they had used micro-USB, I could have made an appropriate cable myself (although this would still require acquiring a micro-USB plug, soldering etc.)
At least with a RJ45, crimping equipment is widely available so I don’t need to worry quite so much about the physical adaptation. Note that RS-232 etc. don’t specify the physical adaption; DB-25 was common around the time of the dial-modem, then the smaller DB-9, and then RJ-45. It’s worth noting that RJ-45 also has much greater cost efficiencies, and with a small adaptor cable (easily are yourself), you could turn a regular Ethernet cable into an appropriate rollover serial-cable (much to the relief of tech-support personnel at the time)
So, to answer your question:
– cost of RJ-45 socket is very cheap
– cost and available of cable components is low and easy
– USB miniaturisation is an on-going process and makes it harder to integrate.
– also, extra circuitry would be required to drive the line-logic and implement USB-serial profile on the device; rising cost. Such cost is pointless anyway as serial-interfaces are increasingly de-emphasised for day-to-day configuration. (Which makes it all the more important to find a suitable cable, or to be able to make one with materials at-hand when things are going a bit pear-shaped.