Because it's an extra step to go to a shitty exchange and convert your BTC to ITNS? That step should be justified somehow. The quality of VPN service through ITNS is likely to be lower than the established commercial services with guaranteed bandwidth and availability. If I want high speed VPN I'll pay to a trusted provider with BTC. If I'm focused on anonymity and don't care about speed I'll just use Tor for free. I'm not saying there's no place for ITNS-based VPN on the market, it's just that market is already pretty crowded and fitting in there is no easy task.