The mishmash of technologies in Battlestar Galactica is hard to reconcile. Some of it, like the use of obvious Earth props (old radios, Citroen cars, old phones) is just a production trick to save budget. The budget was low enough that you may have noticed all colonial paper has the corners cut at a diagonal, this was a joke by the properties department which turned into a stylistic element. Their computers are often a strange mix of modern an ancient.

One part of this makes some sense. The Galactica itself had been turned into a museum, and deliberately used older technology. This is also explained as a clever response to the valid fear that too much computer technology could be compromised by a highly computer-savvy enemy.

Indeed, one can imagine that the colonies, after the Cylon rebellion, could have had something like the "Butlerian Jihad" from Dune, where almost all computer technology was wiped out from fear. However, that clearly didn't happen in the same way, and the use of advanced computer technology in the modern battlestars is explained as the source of defeat.

Still, the creation of autonomous robots is something we're still some time away from, and the colonies don't at all look like a society that 50 years prior was able to build the Cylons, even if they threw some technology away. The ore ship featured in Dirty Hands looks like it's from the industrial revolution. For a ship that has to fly in space, it's hard to imagine why it would not have far simpler automations than were necessary to make Cylons.

In addition, we must consider that this society, in general, has had things like interstellar jump ships for over 4,000 years, and presumably has also had artificial gravity and fancy power sources for a long time if not even longer. It is revealed they understood DNA sequences 4,000 years ago as well, plus kept careful astronomical records.

The most likely explanation is that the colonies had a collapse at one or more points since their expulsion from Kobol. In a high-tech collapse you still have your libraries, but you lose skills and manufacturing capability. Our society is so specialized that nobody can, on their own, make most of the products we use because they all have ICs in them which can only be made in complex semiconductor fabs. Without those fabs we have a long way to fall.

I also imagine that they had a similar fall in the software department. Quite possibly their software systems consisted of large and opaque libraries that nobody fully understood. They may not have had source code to them, some of them might have gone back thousands of years, and all that remained was immense complex code and the description of virtual machine environments to run the code. Today we commonly build software using libraries we never look inside, it's not too far to imagine people working with libraries they are incapable of fully understanding, especially machine intelligence libraries.

I speculate that the colonials were not capable of building the Cylons on their own, from scratch. Rather, they may have had access to old software libraries from AI projects from thousands of years ago. Cobbling those together, they made an intelligent being that they could not understand, that they could never have built from first principles. And they enslaved it, badly.

This explains why they have a mishmash of technologies from different eras. They may not truly understand many of them, but they know how to copy designs and software from their 4,000 year old history. Otherwise, even after a short time, given their technological base, they should have had a technology beyond the viewer's comprehension. Moore didn't want a technology that was only explained with made-up babble, so this makes some sense.