I also think it is a lot more common than we believe it to be, There are more people into ABDL who actually wear and/or use diapers than we realize but as has been said, probably around 90% of them/us spend a lot of time and energy trying to keep it hidden and even when they think they might have seen someone else wearing a diaper in public they are highly unlikely to approach the person and ask.
It is also getting much more common to find people in some combination of LB/LG/DD/MD relationships where diapers are not a thing but one partner acts childish and/or the other treats them that way, even when ageplay isn't really a part of it at all there are many who still have some form of mummy/daddy dom relationship. many of these people may not be into ABDL but may be more accepting of it in others. Some of them may even think they are not into diapers but if they ever try they may get hooked.
Then there are the people who for one reason or another have yet to actually indulge in diapers and explore the ABDL world, be it through fear of getting caught or feeling like a freak for having those thoughts, trying to avoid their desires, simply not knowing that it is possible or maybe just not having the means to buy them.
It is also getting much more known about out in the world, with TV shows, Youtube channels, instagram, reddit, twitter etc. more "(currently) vanilla" people are learning about ABDL and whether they choose to give it a try or not the thoughts may start occuring in their heads.
Then there are the Incontinent folk, now while many of them do not consider themselves ABDL and trying to make a connection between the two can be infuriating to them, but there are a large number of people who come to ABDL lifestyles through incontinence (maybe they come to a site like this for IC support and get exposed to ABDL and begin to gain an interest).
Incontinence is actually much more common than you'd think.
The majority of studies over the years have found that incontinence affects on average around 300m people worldwide. This equates to about 5% of the population. A huge issue is that due to embarrasment, fear and the lack of talk about it, as well as lack of medical records and support in many places (like 3rd world countries) there are probably many more people suffering from incontinence than what is on record
To date, it is estimated that around 7 million people in the UK have some level of urinary incontinence (5-10% of the population). The fact that many do not even see a Doctor means this proportion could be even higher.
If just 1% of those that are incontinent get into ABDL at some point that is at least another 70,000 people in the UK alone (it is probably less than 1% but it's still a lot of people and possibilities).
