It's because it really is an American invention loosely based upon a Thai hot sauce.
The only place I have found it in Taiwan was at some Vietnamese restaurants run by American Born Chinese. I have seem some imitation ones in the supermarket.
Sriracha made its debut in Malaysia approximately a month ago, being sold for MYR 21.90 (28 oz) and MYR 13.90 (17 oz) by Ben's Independent Grocer in Solaris Dutamas.
Pretty nice profit margin there, now that the wholesale price is no longer a secret...