Why did Americans stop at the pacific? Why didn’t they attempt to annex mexico or canada as a whole? I understand that there would be major issues with logistics but that has rarely halted western expansion.
Well, there was a huge amount of American money buying property in Mexico in the later 1800's. Under President Porfirio Diaz there was a lot of foreign investment, as he and his advisers tried to develop the country, and the US was the biggest part of that. The ruling classes had appropriated most of the land and were often happy to sell, and wealthy US businessmen happy to buy . Even Ulysses S Grant , who'd been in the Mexican-American War ( which he felt was unjust) got involved with trying to build a railroad after his political career was over. As a result, a lot of land and most of the mines in northern Mexico were owned by US investors when the Revolution started, and many of those owners had a very dismissive attitude towards the campesinos. That attitude, and those huge business interests, played a very big part in the course of the Revolution. The reformer Madero was removed with US assistance and replaced with a dictator, Huerta. Carranza would try to appease the US and got help in return, that allowed hm to attack Villa and Zapata. And even Villa would, in his early years, try to avoid attacking US properties. So, though the US did not actually try to annex Mexico, it did expand into it.
Diaz famously remarked, "ah, poor Mexico, so far from God, so close to the United States of America".
There are a lot of assumptions baked into the way the question is framed here, all of which don't fit with the historical record. Manifest Destiny is not a well-defined entity, but insofar as it refers to an ideology where the US has both the right and responsibility to colonize, conquer or otherwise take control of territories beyond the thirteen colonies, there are many examples applying to Canada, Mexico and beyond.
Manifest destiny certainly included Canada, at least in the minds of some proponents. There had been a longstanding movement to add Canada to the United States, going all the way back to Benjamin Franklin and the invasion of Quebec in 1775. In the 1840s, the conflict over the disputed Oregon territory nearly led to an invasion of (what is now) western Canada. The famous "54-40 or fight" slogan was a demand for a border well north of what was eventually agreed. However, this conflict was negotiated rather than bitterly fought, at least in part because of the next answer...
Manifest destiny absolutely included Mexico, or at least large parts of it. Prior to the Texan war of independence, Mexico (formally) extended through Alta California and Nuevo Mexico, what is today most of the southwest of the US. All of that territory was annexed, either following Texan independence and the joining of the United states, in the Treaty of Guadelupe Hidalgo following the Mexican-American war, or in the Gadsden purchase shortly afterwards. There was even some talk of complete annexation, though this clearly did not happen. Why exactly that was the case I lack the expertise to answer, though my understanding is that the US was reluctant to govern a territory so large and densely inhabited. Also, that the incorporation of Mexico would have made the existing conflict over the extension or abolition of slavery even more fraught at a time when the US was already teetering on the brink of civil war.
Who said the Americans stopped at the Pacific? The Hawaiian monarchy was overthrown in 1893 with US support, followed by full annexation in 1898. The United States invaded the Philippines in 1899, and Cuba in 1898. Both were turned into quasi-colonies. Guam and Puerto Rico were directly annexed, and still are.
So, I'm no expert on manifest destiny as an ideology or propaganda campaign, so it would be interesting to hear about the cultural side from someone who is. But in terms of the US' actual behaviour, manifest destiny absolutely applied to all of those places.