The main reasons for the EH's huge popularity have been well covered here. I reckon the rub-off from its motor racing successes would have played a key role in its enthusiastic following, particularly among younger Aussies. Don't forget that in 1963 the EH S4 was the first Holden 'Bathurst special' and the stove-hot Appendix J versions morgul mentioned driven by Beechey and Muir were hero cars. For example, the 179 in Beechey's famous 'PK-751' Neptune racer was bored out to 3.5-litres and with triple Webers and the best internals was pumping out 232bhp at a screaming 7500rpm - that was more than double the standard output! Amazing times. If you're interested, years ago I wrote a detailed history of the S4 Bathurst program at this link: https://www.shannons.com.au/club/news/racing-garage/eh-holden-s4-the-generals-first-bathurst-special/