It is being reported that the US has requested Malaysia to "be transparent" and share the known information to eliminate confusion and speculation
The Malaysians' trust in the US may be ebbing as many of the recent stories come from 'US sources', i.e. leaks, and are critical of the Malaysians.
If there is sensitive data you really need to know it's not going to go any further.
The Malaysians may be thinking, 'You're going to blame either our pilots, procedures, or maintenance instead of your US-made aircraft, why should we work with you?'
Would you want to work with someone who may not be able to help, and could just stab you in the back, repeatedly? Then insist that you are to blame as their investigative bodies may conclude (e.g. Egyptair)?
(During one of the investigations I worked on several years ago, a copy of a request for information to a US state containing many sensitive details was uploaded to the state's public online library and made available for all to view. The state refused to remove it . It made relationships rather frosty.)