If you really think this then you don't seem to understand the other factors.
The fact of the matter is is that Lego City Undercover is not a game targeted for your age group anymore, it's targeted to much younger children, and what handheld do you think younger children have? Hint: It's the family friendly Nintendo console. Kids don't care about graphics or the occasional slow down issue, they have much lower standards. I could easily see my eight year old sister loving this, and honestly I think it'd be unfair on a lot of children if they'd just gone with Vita and PS3.
Now maybe it'd be okay if they did another port ALONGSIDE the WiiU and 3DS games, but honestly the amount of money it would take to do probably wouldn't make good profit for Travellers Tales as not as many people would buy it (see: age range and sales figures).