If you're referring to the collectibles, those are normally restocked when a Bell Tree Direct is posted, which is basically a collection of updates and news for the site. The last Direct was a few weeks ago and nearly all of the collectibles were restocked at that time (aside from the cake I believe?), but I'd anticipate there being more restocks on at least the apple over the next month or so. As far as the rest, they probably won't be restocked for quite some time, but you can usually find sellers in the TBT Marketplace who are looking to get rid of collectibles.
I actually do agree, its weird isnt it? . I remember being new, and checking the store, and seeing virtually nothing was in stock. Its normal for those of us who have been here a while, but its odd when you are new.
You will get used to it! Makes hunting for the ones you want all the more fun. (but it does make me an advocate for more random restocks, otherwise its like candy store that only stocks like 2 kinds of candy all the time. Gotta have someting in stock some of the time to be a true "shop".)
We call that supply and demand. I was just going to answer with a simple "because people bought them". Aside from the direct restocks, there are occasional unannouced "secret" restocks. Good on you if you catch them, i guess.