Why GEBs are single premium products
Paul's second point gets close to the main reason.
Since the key to these products is the correct derivative backing, the terms that a company can offer policyholders is crucially dependent on the price of the derivatives, which will be constantly changing.
With the single premium version, a company will agree provisional terms with an investment bank to buy up to a certain amount of the required derivative. The company can then price the product, and assuming the terms don't worsen prior to launch, can then sell up to the agreed amount to policyholders and buy the required amount of derivative backing from the bank.
(As an aside, I know of one company where the terms did worsen between initial agreement and launch. The prudent thing to do would have been to alter the product terms or delay the launch, but instead the company decided to launch the product anyway and put off buying the derivative backing in the hope that the terms would move back in the comapny's favour. Unfortunately, the terms got worse and worse and by the time the company did buy the derivatives, they had lost far more than they would ever make from the product!)
A company wanting to offer a regular premium version wouldn't know what terms it would be be able to get for future derivative purchases, so wouldn't know what terms to offer policyholders. The theoretically correct terms would be constantly changing anyway, making cross-subsidy over time a big issue. You might suggest buying all the derivatives now at known prices, but this would involve making estimates of new business volumes, withdrawals, mortality etc. These estimates would never be correct and so you'd end up with the wrong amount of derivatives and so either have wasted money or have insufficient protection. This would lead to excessive risk for the company and a product that would be too expensive to sell.
Of course, a company could choose to issue a regular premium version (investment houses do offer tracker funds with downside protection of various types), but the risks are far greater and such products aren't covered by SA2.
That's more than I expected to type, but I hope it's helpful.