318 LA roller block questions

So... i have a question... i was recommended a machine shop bout 30 mins away that has great reviews from people near me.. so i'm considering actually just doing the 390 now (in for a penny in for a pound).. what i'm wondering though is, if you can go 390 or 408 (for 360s obviously) without much work why does no one do a 4.25 stroke or bigger? i did some searching and saw that bloomer did years ago but that seems to have died off. Is there a reason? Piston speed issues? or just that no one bothers making the cranks?

P.S. ah.. and after posting this i found Campbell still sells 4.125 cranks, and even Poly stroker kits which i imagine are not cheap :)