With a few exceptions (Flying Jockman, maybe Tornado), normals don’t get pulled in by a specific steam loco or class of steam loco. To the vast majority of normals, an A4 delivers a similar experience to any other steam loco, albeit some are bigger than others. Instead they visit heritage railways to ride behind steam locos, because steam locos are very different to the vehicles used by the big railway and because steam is evocative of a bygone era. Heritage diesels on the other hand, even if they are painted green or blue, do if we’re being honest superficially appear to be fairly similar to the locomotives used on the big railway (yes yes I know its primarily only TPE, Chiltern now). THey offer a broadly similar experience, and make similar noises to the locos they can ride behind on the mainline railway. Units even more so - to the average punter a Gen I DMU or a Pacer sounds and feels much like a 158 or even a 195. So these normals are less likely to be drawn into a heritage railway and pay good money to ride behind a diesel or on a unit when they can do similar in the normal course of their lives.
I have a broadly similar analogy: I crew trams at a heritage location in Derbyshire - the open toppers are most popular, the other vintage double deckers nearly as popular, the streamlined double deckers a little less so and the enclosed single deckers such as Blackpool Brush 630 least popular. I think its because the experience riding Blackpool 630 isn‘t a million miles away from travelling on the current Nottingham / Sheffield / Birmingham vehicles. Whereas riding an open top tram is something one can really only do at a heritage operation.
So its going to be difficult to market diesels or units to normals as most won’t perceive the offer to be substantially different to what is available on the big railway, and won’t [want to] understand the nuances of heritage versus current diesels / units. Heritage railways need to largely leave the diesels and units to the enthusiast market and galas because diesels isn’t where the majority of their business will come from.