A couple reasons it's hard:

1) There's not a lot of room in the engine compartment like there is with a Miata.

2) The ECU on the N/A is very complex and not programmed to manage boost, so you need either crazy engineering or a third-party engine management system. Once you put in a third-party engine management system the rest of the car's computers are unhappy and a lot of work is required to get things back to normal.

3) Few people try because buying and modding an SRT-6 is cheaper.

I think the real reason is 3 - 1 and 2 are surmountable as evidenced by the couple of one-off turbo owners.