There are probably many different factors which i couldn't even attempt to outline accurately, a couple that i imagine are biggies are:
1: Competition: Steam has this problem too, hence the high price tag on new releases on their system. Highstreet and online retailers don't like the idea of a first party system that reduces their sales. As a result i imagine, in order to not upset these retailers, Microsoft would have to sell their downloadable counterpart at a high price thus nullifying the whole point of hosting it online.
2: Bandwidth costs: I can't imagine a full fat DVD9 download being a cheap thing to allow bandwidth for, even on low yeild titles such as the old ones they host now. I can only imagine the amount of bandwidth required to host new, more popular releases would be astronomical. The infrastructure is probably being set up, if only for the next generation of consoles, but i imagine it'll put quite a dent in the bank account so they may be building this infrastructure slowly.
3: Hard drive space: Most 360 owners probably have 60GB hard drives, or even maybe only 20GB. This won't last long if people began downloading full games, deleting a disk install and reinstalling it later is very different to deleting and re-downloading an entire disk. This will likely discourage consumers from investing in a digital copy, thus reducing revenue, and the redownloading aspect will likely add even more to the bandwidth requirement.
4: Home bandwidth: With download restrictions all over the shop on UK broadband providers i can't envisage many people preferring to download an entire game as opposed to pop down the shops/have the disk delivered. I'm on a 10meg virgin line, which is probably faster than the average 360 owner, and i would be very pensive about having to download a full-fat 360 game.
5: No second hand market: This will again hurt the consumers big-time; I remember reading that the vast majority of game retailers' revenue comes from second hand games sales. The digital distribution system, if competitive, will impact the second hand market heavily, possibly resulting in retailers reducing stock of all retail games for that platform (be it as a symbol of protest or to focus on higher revenue systems). This inability to resell a title will also discourage the consumer, prices will need to be lower in order to nullify the loss of this resale value thus upsetting the consumers due to the reasons outlined in point 1.
Obviously these are all just opinion/educated guesses but i wouldn't be shocked if they're not too far off the real reasons.