- There were very few numerical libraries, and those that existed either had a high overhead (compared to, say, calling NAG from C++), or weren't very robust (didn't degrade well in edge cases).
- A windows license for a server machine with > 32 GB of RAM cost an arm and a leg, so server farms got very expensive very quickly. (Oddly, the price of the license depended on the RAM of the machine involved.)
It looks like NAG now has a .NET version, so hopefully things have improved. For a while, it seemed like math in .NET was strictly amateur hour, but I'm sure it's better now. I don't have the strength of will to research if the licensing cost has improved at all.