Because when they all have the big IPL contracts, getting paid peanuts (even the big contracts over here are small in comparison) then why would they?
Gayle was the biggest name we've had over here batting in ages and only came for a few... I can't think of many top quality foreign batsman like the Warners etc that have came over here. There's literally nothing to entice them over here. Their weather is just as good in winter as it is our summer, travelling the other side of the world for 6 weeks is pointless.
In general the test playing players are away roughly 6 months a year, it would take some serious money to increase that time... Like the IPL give but we simply cannot