Why the States didn't buy California peninsula?

The Baja California Peninsula (English: Lower California Peninsula, Spanish: Península de Baja California) is a peninsula in Northwestern Mexico. It separates the Pacific Ocean from the Gulf of California. The peninsula extends 1,247 km (775 miles) from Mexicali, Baja California in the north to Cabo San Lucas, Baja California Sur in the south. It ranges from 40 km (25 miles) at its narrowest to 320 km (200 miles) at its widest point and has approximately 3,000 km (1,900 miles) of coastline and approximately 65 islands. The total area of the Baja California Peninsula is 143,390 km2 (55,360 sq mi), roughly the same area as the country of Nepal.   The peninsula is separated from mainland Mexico by the Gulf of California and the Colorado River. There are four main desert areas on the peninsula: the San Felipe Desert, the Central Coast Desert, the Vizcaíno Desert and the Magdalena Plain Desert. 

Interest in California was driven by manifest destiny, the gold rush and fur trade, none of which included Baja. There was no reason to include it in the cession. 

Baja is a desert.  Nothing there.  No Gold.

It was a mistake not to purchase everything West of the Colorado (Yuma)

I am not sure it was ever for sale.  We took what we thought we owned from some other purchase or agreement, and Baja was not included in any such sale or treaty.  Why there was not such a forced transfer?  It wasn't seen as worth the effort at the time.  It was left for a future theft or purchase if such theft/purchase was seen to be worth the effort (cost).  Evidently, no one in power ever got enough support for any argument that the territory was worth anything that would be worth the cost.  It wasn't wanted, basically.  It isn't a question of why it wasn't bought or taken, so much as why no one wanted it, and that is because there really is not a whole heck of a lot there to want.  It is not as though American settlers were streaming into the territory because it was such a fantastic opportunity.

You can guarantee that if Americans were migrating there in numbers, it would have been taken whether or not Mexico wanted to give it up.

I don't agree with "skeptik" that the Treaty of Guadalupe Hidalgo included the cession of Baja California.  The treaty fixed an east-west boundary about 30 miles north of where the previous border of "Alta" Califonia had been.  Probably Mexico wanted some way to access Baja California, even though there's no "there" there.

There was already enough room for all the crazies in California although it seems to me that they may need more room soon.

It is not for sale.

El Diablo Rojo.

Actually, the Humboldts are appearing off the coast of Southern Oregon...but climate change is to sell books.

Manifest Destiny was the idea that the nation would grow to cover the width of the continent.  That included the west coast of the US, not Baja California.  The original purpose of this growth across the continent was to get new land to grow more cotton.  But you can't grow cotton in Baja California.  In fact you couldn't in Texas either, which was why it was its own separate country for a while.

Anyway, we didn't buy California, we just took it.

Too.many mexicans. They wanted it.