For that matter, why didn't the boy remember his (previous life's) real or assumed name?
I don't like videos for this sort of thing, they're sort of an unchallenged (and unchallengeable) Gish Gallop for claims that need challenging. The best I can find otherwise, with a cursory google search, is
this article from NBC News, a bit over two years old. It's not much more informative, but does contain an excerpt from the book by Dr. Jim Tucker, which says (to me) a lot about the methodology behind the "science," and has this bit-
Ok, fair enough; but, after admitting that "Ryan was off on some of the details," Dr Tucker then goes on to list the matches he believes supports the boy's claims. It's hard to tell from the wording whether the claims as set forth by Dr Tucker are what the boy (Ryan) actually said or Tucker's interpretations, but, either way, they're still not all that impressive. In fact, to me, they sound pretty much like what you could expect of someone active in the Hollywood scene of the time- being a dancer first on Broadway before going to Hollywood was probably not an unusual career path, people changing their names was certainly common, and, gosh, sunbathing and taking girlfriends to the beach? I bet Martyn was the only one who
ever did that in LA.
So, all in all, even though Dr Tucker makes a point of saying he wasn't "trying to see if there was anyone whose life matched Ryan’s statements; we were looking to see if Marty Martyn’s did," all he did in the end was match Ryan's statements to a typical (even stereotypical) Hollywood life- he did indeed only match them to "anyone."
So...why couldn't Ryan speak Ukrainian or remember his own name from the previous life? I'd bet the answer to questions like this would be along the lines of "well, we don't know how reincarnation works exactly" (it works in mysterious ways, you know). When all is said and done, the methodology applied here appears to be to accept as criteria for matches what would identify a range of people, but not to demand a criteria specific enough to identify an actual person. I'm no scientist, but if your criteria is only strict enough to form a hypothesis, but too loose to ever test it, then it seems to me you're doing more faith than science.