It's a new vulnerability class. Prior to Spectre, nobody thought that code which didn't execute (and couldn't execute) could affect architectural state in an observable way. It's hard to overstate how bizarre the vulnerabilities from the Spectre family are from a software point of view: it's leaking data from code that not only didn't execute yet, but also can never execute, and in some cases doesn't even exist! It's like receiving a packet your future self sent to the past, except that your future self had been dead for two years when he sent the packet, and for some reason he's actually a parrot.
Once a new vulnerability class is discovered, researchers will start looking for new bugs in and around that class. Which is why we have seen lately so many issues disclosed around speculative execution and data leaked through shared microarchitectural state.