I have no doubt that there are very talented pilots who went through the T-1 track and went on to fly heavies, and who would likely excel if they crossflowed to a fast mover.
But let's remember that the AF training pipeline has always been a game of numbers, and Big Blue has to select the pilots who have the highest likelihood of making through IFF and FTUs in the time directed in the syllabus. I'm sure there are plenty of UPT washouts who would have made great pilots if they'd just had a couple more rides, right? But, as we know, the syllabus directs when certain learning milestones will be achieved, and that is Gospel for the training pipeline.
The brass who are making these decisions are old enough to remember the last fighter crossflow program in the late 90s and how generally sorta-slightly-below-average that turned out. Yes, we're talking about something different now than then (e.g., now the question is T-1 vs T-38 trained, but then the only folks eligible for crossflow were T-38 UPT-trained pilots), but because of this I think the lessons of the 90s crossflow are even more amplified now than then.
I've posted about this before (several pages back in this very thread), but for those who didn't live through it, the fact is many crossflow pilots didn't end up performing as well as hoped at all stages of follow-on training (IFF. FTU, squadron MQT, etc). Some of them did great, of course (I know a couple that went on to perform well above average where I was in the F-15E community), but statistically they did "worse" (in terms of pipeline training washouts and issues in operational units). Remember, during this crossflow program it was only T-38-trained pilots who were eligible.
Most of the dozen or so crossflow pilots that were my IFF/FTU classmates and later follow-on squadronmates were superb officers with fantastic officer performance records (and extremely good dudes to boot), but that didn't always continue into performance in the cockpit. It wasn't a talent issue with the crossflow pilots so much as it was an experience issue; one has to acknowledge, weather it is politically correct to or not, that there are significant cultural differences between the fighter community and other flying communities (although the bomber community is a somewhat close relative) that translate to differences in skills/airmanship in the pilots that come from those communities. What makes an aviator great in the MAF isn't the same thing that makes an aviator great in the CAF.
On the most basic level, the crossflow pilots, for the most part, were not used to being single-seat decisionmakers at much higher speeds, and much higher Gs, while hand-flying significantly more aggressive/dynamic maneuvers. Many times the core airmanship just wasn't operating well at 400 knots and pilots were just behind the jet (sound judgment, just not fast enough); sometimes a thousand hours on autopilot in the flight levels did not translate to having hands good enough for even basic admin formation work, much less more complex BFM or surface attack. This isn't unique to the crossflow folks, though; this is the same thing seen many times with ANG/Reserve fighter units that hire non-fighter guys and send them through IFF and fighter FTUs. There was a big wave of those guys back in the 2003-2005 timeframe (mostly A-10 units at the time, but I don't remember why), and they had an unusually high washout rate, too, with the guys who did superb being the exception rather than the rule.
None the less, the end result was that there was higher attrition of the crossflow guys compared to straight pipeline students, and the fighter brass largely decided the crossflow program wasn't that much of a benefit. Again, not that the crossflow pilots were idiots or anything (in fact, quite the opposite -- most of them had impressive OPRs/jobs/awards, seemed to have been superb pilots in their previous lives, and were really great dudes), but their previous flying time had given them habits and airmanship that did not dovetail into success in fighters.
And all of this was with pilots who had 100-ish hours of training as a fast jet single-seat flyer and decisionmaker before going to a multi-pilot airplane. Now, how do you think that learning curve is going to be with a dude whose only single-seat judgment and decisionmaking was Phase II in T-6s however many years prior?