a lot of vendors do not do registered sal, its a bit of paperwork and an extra cost they just dont want to spend time on. if you want tracking they usually just push you to ems. it does bring 6000 yen insurance with it (along with tracking) for the 410yen cost.
to the US sal has never let me down over hundreds of packages in like 15 years, so ive never really cared about registering it or even doing ems much as im very, very well self insured now that a loss of a package is no issue if it were to happen. If you are willing to put the risk up front on the first 5-10 packages usually by then you are self insured for a regular $50-150 package to be lost down the road. then just builds quickly not doing registered or ems. of course you have to have patience for your trains to wait 2 weeks... ;-p
i just noticed something on sal that the shipper can get bulk discounts on sal shipping, so i wonder if some of the odd little delays that happen with it might be the shop or shipping agent holding onto packages until they have enough to get the 10 or 20% discount?
actually in the last decade i have only lost one package at all in the mail and that was w/in the us, bunch of japanese magazines where the package exploded along the way. it was reported and months later the bit of package with the label got to me, but alas in that sort of situation the magazines were probably shredded on a conveyer somewhere and toast.