ISAs are great, but they're accessible. If you've got the self discipline not to spend built up savings then they'd work very well as a retirement fund, after all the income is completely tax free unlike pensions. But people don't and that new computer/holiday/car/house that they want will just come straight out of the pot.
Surely the other difference (which works in the favour of pensions saving) is that contributions in to a pension are tax free - where as contributions in to an ISA are not? Pensions are taxed on the way out, not on the way in - but it's the other way around for an ISA. My ISA savings come from my net income, not my gross (as for my pension).
Does any of that make any sense?