My speculation is twofold: first, the 6-subs have been determined to be quite wasteful of precursors, as their yields are far lower than the 1-subs, and the synths are more difficult. The producers of ETH-LAD and AL-LAD stated some time back they would not be making more because the profit margins were too narrow or something. That said, someone did make AL-LAD again since it was so popular.
The other reason, I think, is that generally, isopropyl groups are wild cards on psychedelics, causing unpredictable changes to what you might expect from the molecules. So they're risky, especially with difficult and low-yield (and expensive) lysergamide synths. Take 2C-iP... it is very impotent, with generally no real positive reports and some negative pones. Seems like a dud. It was available for years and there are only a few reports. DOiP is less of a dud, but Shulgin himself stated that he took it up to 4mg and had absolutely no effects. I have some and me and some friends have tried it, and all of us get effects, but I find it rather jittery and much less good than other DOXs, and also entirely non0-visual. Some of my friends liked it better.
Now, look at 2C-T-4, which is 2C-iP but with the sulphur bridge of the 2C-T-X family. 2C-T-4, instead of being impotent, is extremely potent, and has unique effects, some people find it one of their favorites, other hate it. Most describe it as somewhat dissociative. it's also very long-lasting, unlike 2C-iP and DOiP, which are shorter than their related brethren.
And finally, look at DiPT... this one is the craziest of all, as it produces a profound and unbelievable shift in hearing rather than visuals. It's like DMT for your ears. That's just so strange! And unpredictable, who would have guessed? And MiPT is unus7al too, though not as much so.
Anyway my point is that isopropyl groups are strange, and kind of a crapshoot. In the case of complex lysergamides with difficult synths, the risk probably outweighs the potential reward. And Shulgin never made isoPRO-LAD in TIHKAL so there is nothing to go off of, unlike the other 6-subs we're seen.
I expect we'll see PRO-LAD before isoPRO-LAD. But I hope we see them both!