Titanium is very common in the Earth's crust. But turning titanium minerals into pure, ductile titanium metal is more energy intensive and complex than making most other metals:

https://chem.libretexts.org/Bookshelves/Inorganic_Chemistry/...

At the time, the USSR had already invested significant resources into making titanium for submarines. Buying titanium under false pretenses was easier than increasing the domestic industrial capacity for making titanium, particularly when the plane was not going to be produced in large numbers.

It's a fairly well known and documented story, do you have anything similar in evidence rather than just conjecture?

I think it's mostly anecdotal (I have heard it too, but I can't think of where)

Not a metallurgist, but if I had to guess it was hard to find in large enough quantities at the time. I also understand that it's a lot harder to work with than say aluminum or steel

Speaking of which, King Louis served his guests with aluminum silverware, since at the time it was more valuable than gold due to the difficulty of extracting it at the time, despite its abundance.

Why deplete your own supplies when you can use your opponents?