Because especially in its infancy .NET was quite slow for the machines of the time (they somewhat fixed that up in later releases); it also meant a full rewrite

Because especially in its infancy .NET was quite slow for the machines of the time (they somewhat fixed that up in later releases); it also meant a full rewrite most of the time (COM interop is well supported, but still rather nightmarish at times). Given the paramount importance Microsoft gives to back compatibility, I understand why they took their choices.