Despite being professional for about 15 years now, English rugby is still built on an amateur foundation. This is especially true of many of the grounds. At least four of the premiership clubs still share a grounds with an association football club, and many have extremely primitive facilities and limited seating. You also see this legacy in the current fuss over whether or not London Welsh should be promoted to the premiership. Games at Twickingham suffer from no such disadvantage. In short, the English game hasn't completed the transition to a fully professional sport in facilities, marketing or administration. Not really a surprise -- for example it took American football about 40 years to make a similar transition from what was primarily a college sport, to a professional one.
A lot would also depend on the ground. I've watched a number of games on television and most of the grounds look pretty full. But the grounds themselves dont look big. By the look of it many of the grounds were built not expecting the numbers that may want to attend.
Because they are no longer a force in club rugby. That would be Ireland.
