From a publishers perspective:
Marketing: Can throw up to and inclusive of 360, PS3 and PC versions into the same marketing campaign. Marketing can easily cost as much as the actual development of a title. Wii versions usually require their own marketing effort. This is a considerable increase in cost because they cannot slap some PC version footage or whatever with the Wii version in many places without being fined for misleading advertising.
Publishing: The average sale price for Wii titles is lower than Xbox 360 and PS3 titles. So whilst they may save money in making it, they don't exactly reap that reward in a retail setting. Theres a very good reason why Wii games recieve quick price drops and big stars like Just Dance initially retailed for $40. This means the actual costs of royalties, packaging, licencing to Nintendo, shipping are much higher, especially as Wii games aren't front loaded so therefore must be continually drip fed into stores whereas 360/PS3 games often only require one big production run and shipment and they can forsee demand better due to pre-ordering.
Long term sales: Often a publisher has 5-10% of its revenue gained through legacy titles. Will Wii titles stand the test of time as compared to many HD console releases? Also 360/PS3 titles often sell the same or better when made into sequels whereas on the Wii especially sequels have often done considerably poorer. Many titles are selling as well or better on their 3rd or more installment.
From a developers perspective:
UE3: For medium to smaller developers who made Xbox 360/PS3 titles it was a great boon for them to be able to use Unreal Engine 3, Unreal Engine 3 doesn't work on the Wii due the Wii's architecture and lack of RAM not meeting the minimum requirement. Immediately any developer wanting to use UE3 cannot make a Wii title.
Development: Xbox 360 and PS3 and PC use the same shaders, they can also be ported between relatively easily due to similar capabilities and architecture due to Microsofts efforts on the 360/PC front and the fact that the Xbox 360 and PS3 are cut from the same cloth and are more similar than any consoles architecturally in the recent (20 year) history of consoles. They also don't require significant developer attention as analogue to mouse conversion is well known whilst developing a workable Wii control scheme would take the limited attention of the core development team.
Desire: Finally developers like the HD consoles better because they are artists and they work for the love and certainly not the money. Publishers know this, which is the foremost reason why they don't give their best teams to the Wii because their core talent can and will walk if they don't like something. Artists like being able to express their artistic work and the 360 and PS3 lend them this ability more so than the Wii by a considerable margin. These people could earn considerably more if they worked in other industries. If it was just a job they wouldn't be doing the job of working on the Wii by choice, the ones who don't have a choice work on the Wii, hence shovelware.
Tease.
