I'm sure there are reasons why; it'd be interesting to hear what they are rather than just "there are reasons".
TV is very powerful for ads that require or leverage immersive, experiential pre-visualization. These are expensive to produce, so not fit for micro-targeting. they are also large data-payloads, so not fit for distrubution in N-bundles.[1]
At some stage, in the Steve Jobs world (no pun intended), the mediums will converge, technologically. And then things will change, a bit. But the above forces are somewhat fundamental, and just need to be considered (at least).
Its generally not the TV people, they are more caught up on exclusivity around content. Its the advertisers, who sense the ability to communicate online is (has not been) up to the standards they need for the black arts in which they are well versed.
Other people can probably add more to this, but this is at least a roadmap for analysis.
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[1] Questions also remain outstanding on the environment of online: is it too distracting/evasive, will it just be skipped, etc. But this is a grey area, moreso than the others. For example, humour or design applied to add interest might be a strategy (if it were economical to run online-spefic ads, which is generally is not yet for TV-replays). The other evidence (as noted in another comment) is the reduced payload. Hulu would be putting more on if they could, but we can infer they don't think its a good idea. Most likely, the internet consumer is not willing to tolerate the additional adds before leaving or being so distracted, the existing ones lose their value. Etc.
Ads on Hulu are and have been more effective for advertisers than its television counterpart (http://socialtimes.com/hulu-ads-are-twice-as-effective-as-te...). The issue for Hulu is that because there are less of them in every stream there is less to go around. This especially bothers the content providers.