There are lots of problems.

First the standard vertical rack wouldn't work - you would not be able to remove any blades from the rack since they are submerged.

You would not be able to lift the entire rack out of the tub, since you might not have enough height, but worse the other computers in that rack would probably burn up from a sudden loss of cooling.

So you would have to switch to horizontal racks, so you would lift one blade out without affecting the others. Of course that wastes tons of space. If you want lots of servers you would need multiple racks one above the other - but then how would you service the upper ones?

And you are wasting space again - you need air height above each rack. Normally in a vertical server, the gap you need to remove the server is the same gap you need to walk there, so no extra wasted space.

And then there are weight issues - a typical 21.5 inch x 19 inch x 8 feet rack would weigh about half a ton just from mineral oil! (Not including any metal.) Good luck building something strong enough to hold lots of them, and forget mounting them one above the other.

All of this can be designed around, but it's not a simple matter - for now you would need to make it custom.

It is not prevalent because they are still testing it and the oil servers are not yet available for purchase. I do hope when they do become available that the price is not too high.

The articles around this are interesting and consider how it could become prevalent and they rightfully point to high performance computing. I think another obvious area is countries like India which have very high costs for power and do not have much in the way of existing datacenters.