Because all of our bowlers are better at forcing wickets than keeping the run rate down. It's most obvious with West and McCallan, because they're flight bowlers who look for turn. Boyd Rankin too, if he'd had ten overs here instead of four he'd definitely have nicked one or two out the way he's been bowling.
Trent Johnston is probably as good in both, and Alex Cusack is the one bowler who's better at T20. But the best example is Peter Connell, who hasn't even been playing this tournament. Pretty unreliable but almost always picks up an early wicket or two and can destroy sides on his day.
Also we have a habit of sending down an over of looseners first up. Obviously more costly in T20.
Then there's our batting. We bat very low, and everyone's fairly capable, but only a few can clear the boundary with ease- as was obvious against Sri Lanka. There are some required strike rates we just can't possibly do. Only the O'Brien brothers are suited to T20, everyone else is much better when they can play their way in and 100 is considered a good strike rate.
Sadly, i don't think we'll have the opportunity to show it on the world stage. The next world cup is in India, and we seriously struggle on slow, low pitches. By 2015, who can say where Irish cricket will be?