It's been hinted and spoken about throughout the history of racing games that Ferrari can either be difficult or just outright expensive. Rumours of specific requirements and whatnot which can make securing them tricky.
Straight4 have clearly been somewhat limited on budget, especially when it comes to licensing as things like Northampton (Silverstone), Derby (Donington), Lexington (Mid-Ohio), Brianza (Monza), San Marino (Imola) would suggest. And a good number of the cars announced have what I'd call semi real liveries, real designs but fictional sponsors. That kind of tells me that there's been a financial balancing act when it comes to paying for licenses for circuits, manufacturers, logos etc so perhaps Ferrari was just one expense too far for them.
Cost example, allegedly Acura charge $50k for the ARX-06 alone.