Hello Ian and welcome.
You are right that it does not make sense that LRB's are generally quite inexpensive. I think it has to do a lot with supply and demand. The supply is huge and the demand is small compared to how many LRB's are available. Personally, I can't understand why people even collect boring modern coinage, but in the US people go crazy over coins only 100 years old and put them in plastic slabs and worry endlessly over what grade their coins are, sending them off to companies for grading and then if not satisfied sending to another company in the hopes of a higher grade. Numerous books exist on US coins just to let one know what the current market price is for each coin. Many people do not even realize that you can easily collect ancient Roman coins, or for that matter, that they are even obtainable. For about 15 years I mindlessly collected a variety of US coins, that were for the most part identical except for the date or mintmark. One day, in a US coin magazine, I saw a small add for an ancient Roman coin for sale- a Caracalla denarius. I was hooked and twenty years later I could not imagine a better way to spend my free time.
Of course, some people might stay away from ancients because there is a bit of a learning curve. I like to think that ancient coin collecting caters to the more intelligent collector, but, in this, I surely have a bias. However, ancient coin collecting is a multidisciplinary endeavor that encompasses many fields like history, linguistics, archaeology, and science. This might be daunting to the average person, but I like to believe that most ancient coin collectors are above average.
Among ancient coin collectors, there are also a few that seem to think of LRB's as a lesser area of collecting. This is an uninformed and ignorant opinion, but it does show up on occasion. Here is a topic about one such recent example-
http://www.lateromanbronzecoinforum.com/index.php?topic=605.0 I am divided on this topic though, part of me thinks it is strange that more people do not collect LRB's, but the other part of me is very glad that more people are not involved. More people competing for coins would drive prices up, like the Queen Victoria pennies you mentioned and eventually this field would be just as commercialized as the modern coin market.