A lot of reasons - one, we only recently (in academic research time) started using single servers to host services from multiple customers, so the value of these sorts attacks only recently became apparent.

Second, as I understand it, Spectre and Meltdown really started this whole parade because prior to those vulnerabilities, speculative execution attacks were something only academics ever talked about - everyone assumed it would be too difficult to pull off in the real world. When that received wisdom was proved wrong, it probably opened the floodgates for researchers - both in terms of intellectual interest and money.

Also, re: why Intel and not AMD... I think Intel is probably a higher-dollar target due to their dominance in the server market, but also probably because they have been neglecting QC for years... see, e.g., http://danluu.com/cpu-bugs/