1. Expense; you need a new hood, fenders, bumper and lights. These are big items, and they need to be shipped from Japan. Even using sea freight it could easily be $500-1000 in shipping and import fees alone.
2. Availability. These cars are rare - I have seen one in person, and one other for sale in Aus, in 4 years. They also aren't that popular in Japan; not nearly as much as the Celica was. Also, since you need the whole front cut of the car, you may need to actually buy the whole car to get what you need. They had boring engines, so buying half cuts isn't practical.
3. Looks. Some people love the look of the Curren, others don't. Personally, I'm on the fence.... I don't think it looks bad, but the design is very 'same-y' for that period of Japanese cars. You have to remember that the people that would be paying all this money for the conversion probably quite like the 4-eye look of the car from the factory, so it isn't as big a deal for them as it would be for potential buyers. Also, the Curren only came as a coupe, which suits the styling. Curren front with liftback rear looks a bit funny.
All in all, a GT4 front, or SSIII lips and projectors ends up being a grand or two cheaper, easier to source, and IMO better looking.