Why input credit not available for Construction service

I have a query.  For both industrial and commercial construction services ST is payable on 33% which roughly works out to 4% .  However, you cannot avail input credit if you avail the abatement.  However, after introduction of Works Contract services, for the same commercial/residential construction you can pay ST at 4.12% of the value but still you can avail the input credit.  My question is what is the logic behind denying the credit under abatement regime when the rate of Service Tax is almost same