Well seeing as how many other companies copy Apple's design aspects and features of their OS and softwares I would say since other browsers don't offer Top Sites it's not a "must have" feature. There's a reason Apple removed it. That's probably the reason.
It's not about being a "must have" feature. It was not extremely special, or unique anyway, it's not even that far from having a row of "Favourites". It was uniquely presented one could say, although it had some glitches and occasionally failed to get a nice thumbnail of a site.
Problem mainly is they just threw it away, without any notice, without some migration/export/save-as-bookmarks prompt or anything at all. We just launched a browser to a completely new 'Start Page' (kinda ripped off from the current Chrome and Firefox btw) which actually had none of the sites I was used to seeing at launch, just some random 'Frequently Visited' thumbnails and some Favourites. My process? => To the garbage. My muscle memory => garbage.
Not the end of the world, obviously. Not a nice move from Apple, either.