Titanium is not at all rare 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 effort 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.