2 main reasons:
In NA, the 360 isn't selling bad to require a price drop. MS is making around $70 per premium 360, and is recouping alot of their launch costs. In Europe, its selling well enough to not "need" it, but did drop to 199 euros for the prem in Germany (a poor selling market). The 360 core in Japan as well as premium are also cheaper than any comparible market. So MS has dropped the price, just not majorly in the US.
There isn't enough "must have" titles out to justify a demanding price drop. MS dropped the US price $100 after NPD reported just 80,000 units moving in April. The 360 has yet to go below 170,000 in any month.
I am CERTAIN once August or September roll around, MS will institute a huge price drop in preparation for Halo 3 and GTAIV. Bill Gates already said they know Christmas 2007 will be the largest, and it will be. In all actuality, in Christmas 06, you could find Premiums for ~$300 USD, as some stores sold them with $100 off gift cards - so it's easily feasable for MS to reduce the price.
IMO, MS is waiting for the 65nm process to come to market, and the old(er) models to sell out in June/July, as the 65nm process should lower the price *around* $20 a unit. Therefore, MS could easily sell the premium at $300 and MAKE A PROFIT. Good strategy if they want to make money. Of course, MS could loss lead and do $250 for the premium and $150 for the core. Now that'd be crazy....$150 for a core this year.