difficultpickle I can think of several actually. Mind you all are speculation so please take with a pinch of salt!
For a start, the Malaysian government have got experts coming out of their ears helping them currently. IIRC there are experts from NTSB, the UK air transport people, the French air people (can't remember the correct names for the UK and France, sorry!), Inmarsat, Boeing, RR in KL. I think the FBI is there too (and it wouldn't surprise me if the CIA have got a finger in, or is that tin hat territory?!). Presumably they are also getting (some) data back from the various governments. So I think they're absolutely drowning in help and data at the moment. Now if I were to put my IT projects experience hat on (not comparable to this but the closest analogy I can come up with!), if I was in a shit situation, I would prefer to choose my experts, not just take on every expert who offered to help. It may be that down the line I would need to get more expert help, but I would prefer to make that decision as and when the situation arises (otherwise too many cooks come to mind).
Or, possibly the offer for help has not gone through the proper channels. 'Face' isn't the only important thing, so's protocol.
Could be political - a lot of Malaysians (who of course make up Malaysian agencies) are automatically suspicious of the US so any offer to help may not be jumped on immediately.
Or, the Malaysian government don't want too much outside help because they want to have tighter control over the findings.
Or, the Malaysian government think that local agencies are up to the job and don't require input from outside agencies.
As far as I can tell, it was the "legal and intelligence" agencies whose offers for help were refused. I think the comment from the source on the NYT is quite telling actually: "We just don’t have the right to just take over the investigation." Err, that's right, you don't. You can help, but it I should think only a very malleable nation would allow US investigators to completely take over an investigation of this magnitude, regardless of whether Malaysia has the experience or not (that whole face thing again).
AngelaDaviesHair I completely agree with this: One thing I really don't believe: that the Chinese regime gives a monkey's toss about 152 dead ordinary Chinese citizens and will stop at nothing to find out what happened to them. The Chinese govt would bump off that many of its own people without a moment's hesitation. They have apparently searched their airspace and moved satellites to look for the plane but offered zero transparency to the Malaysians in the process.
Which is why I find the Chinese insinuations and accusations so infuriating.