SARS-CoV2 is quite similar to the previous SARS virus, so the development for vaccines against that one could be reused. In this case the target was already known, every vaccine is targeting the Spike surface protein. So the existing knowledge allowed them to mostly skip the very first phase of development.
The mRNA platform the BioNTech/Pfizer and the Moderna vaccine use is new, and that is generally something that can lead to shorter development.
As far as I understand, the biggest difference here is simply doing more things in parallel that you usually would do sequentially. This adds more risk because you already waste money in later expensive steps that are unnecessary because a previous step turns out to already fail the vaccine candidate. The easiest example here is producing the vaccine before phase III trials are completed, that is pure risk (in part assumed by governments in this case). This is really a case of "money is no object", a vaccine is useful enough in this case that you can take a lot of financial risk and pour lots of resources into development compared to a less critical vaccine.
The other thing that the more pessimistic timelines assume is that not everything will work out. Any problem can delay a vaccine or kill a candidate entirely.
The mRNA platform the BioNTech/Pfizer and the Moderna vaccine use is new, and that is generally something that can lead to shorter development.
As far as I understand, the biggest difference here is simply doing more things in parallel that you usually would do sequentially. This adds more risk because you already waste money in later expensive steps that are unnecessary because a previous step turns out to already fail the vaccine candidate. The easiest example here is producing the vaccine before phase III trials are completed, that is pure risk (in part assumed by governments in this case). This is really a case of "money is no object", a vaccine is useful enough in this case that you can take a lot of financial risk and pour lots of resources into development compared to a less critical vaccine.
The other thing that the more pessimistic timelines assume is that not everything will work out. Any problem can delay a vaccine or kill a candidate entirely.