Your money never is actually invested in the market (unlike Variable UL). The carrier puts your money into its general account.I thought IUL allocations allowed a portion of the premium to follow the growth of the s&p 500. What would a low interest rate environment have to do with that, and what do bonds have to do with IUL's?
So, if you put $100 into an IUL, the carrier will take 95 dollars will buy bonds, $1 will go to commissions, admin, etc. and the other $4 buys options to give you notional exposure to whatever index you're tracking.
If interest rates go down, the carrier now needs to put $97 into bonds, $1 to the house, and now they only have $2 to buy options. That means lower caps, pars, and higher spreads.
Lastly, options are primarily priced using two functions, volatility and interest rates. So not only does the carrier have to deal with the problem outlined above, but low interest rates and high volatility makes options more expensive. This further magnifies the problem.
Hope that helps.