Mark, why do you suppose that is? I kinda noticed that myself, although I come from a different lineage, but also coming out of the Stockton scene.
I have also been involved in Wing Chun for a long time, and one of the other senior WC instructors in our association is also a Modern Arnis instructor. A nice guy and very dedicated martial artist, but in the week we were training WC together at a seminar, we never compared notes on our FMA. We were training WC intensively and didn't have a lot of free time, but I also sorta got the vibe that he was content with what he did so there wasn't much motivation to exchange ideas.
To be honest I think and this might get people PO'd at me, but I think it is several things.
1) Cult of personality, both followers of Dan and Remy generally are fiercely loyal to them. At some point long ago (I believe it was probably back when Remy first came to the states in the mid to lat 70's) something happened where they didn't mix. Over the years I've heard it from 3 or 4 people from Dan's side that for some reason Dan didn't like Remy. Now these people didn't have any connection between them and it's over the span of years, just comments made during conversation, so who knows.
Likewise when I trained with Ernesto (Remy's younger brother) I was also seen as disloyal to Remy by Remy followers, yet Remy and Ernesto didn't seem to mind in fact they encouraged it. So maybe it's seen as dislolayity to students and they discouraged it and said there was an issue who knows. Truthfully it doesn't really matter now.
2) Protecting their meal ticket. Both guys competed for the same market.
3) Dan had a school, Remy didn't. Dan naturally had a base from which to work from and Remy had people all across the USA doing FMAs (Modern Arnis part time). Worse yet many of these people were Karate, TKD, students first and doing Modern Arnis part time. If you remember back in the 80's many styles didn't mix, and Dan's people didn't mix with Karate and TKD people to well because well their arts were useless instead of useful
4) My style is all you need syndrome. There is the belief that the style you are studying is all you need, therefore why look anywhere else? I believe this is a big part of it, because I find this a lot in the FMAs. We have over the past 4 years had a cross training group in the DFW area where we try and have different instructors teach and get together with other FMA schools. All styles are on equal footing, all proceeds split evenly between presenters. We've reached out to JKD Kali people and not one instructor has responded, well one has one time. But we've had several other instructors come and share. Now we'll get fliers about other seminars they are hosting but they won't come to our events. Sadly PTK was the same really. Even with me training with a PTK class on the side couldn't get them to a event like this.
Don't get me wrong I'm not trying to put down JKD Kali, PTK etc. etc. I'm just saying that as one long time FMA practitioner told me not to long ago he was taught not to look at other styles Serrada was all he needed. Whereas when I met Guro Dan in 82, he was the inspiration behind me checking out everything. So I think it can be a mindset that is introduced into the student on purpose or not from the instructor.
5) Remy was probably seen as sub standard as a teacher and a stick fighter. Look at Remy's videos compared to Dan's. TBH Remy's videos, his 80 series, were not all that impressive to me back then. Having trained under Remy and having a better understanding of his system I have a completely different view of them now. Same thing when Remy was teaching at the seminars and Dan's instruction. Remy's seemed basic compared to Dan's. Yet again the more I got use to the instruction my view changed.
So long explanation short, both camps I think have a different view of each other's teacher and art. Both sides perhaps feel superior or comfortable with their style and don't feel the need to share or try each other's system out. That's my on it anyway.