Well, just look at a map from the period that England tried to start colonizing (early 17th century). Most of Latin America and South America were already conquered by the Iberians when Britain got their first successful colony at Jamestown in 1607. The Aztecs were conquered by Cortez in 1521 and the Inca were beginning to be conquered in 1532 by Pizarro. Geography was another reason, they were just a lot closer to it.

As /u/PrelateZeratul already stated, the Spanish and Portuguese empires were already well underway in their colonial endeavours by the time the British got started. The Treaty of Tordesillas authored by Pope Alexander VI in 1494 divided all future colonial endeavours down a meridian in the Atlantic, with Portugal taking everything to the east (i.e. Africa) with Spain taking the newly discovered Americas.

Unbeknownst to either party at the time the treaty was signed, the meridian intersected Brazil's Nordeste region which enabled the Portuguese to gain a foothold in Latin America, but beyond Brazil literally everything in the New World was ceded to Spain before it was even explored, discovered, or claimed by Spanish explorers. When the Iberians began colonising en mass they generally did so either in the metropolitan centres of the most advanced New World civilisations (e.g. the Aztecs in México, the Incas in Peru), in places with extensive mineral wealth (e.g. the silver mines of Potosí in Bolivia, the perceived extensive gold reserves of the Taino peoples in the Caribbean), or areas where they could grow the spices that they were originally seeking (e.g. tropical areas).

Britain and France were late to the proverbial party and had to settle for the unexplored northern regions of the Americas. That is not to say, however, that their colonising efforts were necessarily focused primarily in present day Canada and the United States. The profitability of sugar made tiny colonies like Barbados and Jamaica more lucrative than British North America for some time and the French colony of Saint-Domingue (present day Haiti) was said to be the most valuable colony on the planet. When you compare Hudson Bay Company traders traversing thousands of kilometres of uncharted territory to trade furs with the natives with sugar plantations using chattel slavery it's not hard to understand why the British and French were so invested in the Caribbean.

As the British and French maintained interests in the Caribbean, so too did Iberian explorers conduct several surveys of North America. Spanish explorers were amongst the first Europeans to enter the Salish Sea in the Pacific Northwest and a couple forts were established on Vancouver Island, the Viceroyalty of New Spain extended far into the American Southwest, the territory of Louisiana encompassed the Mississippi watershed all the way to the Canadian border, and Florida was a Spanish territory for quite a while.

In short, I would say the Portuguese focused specifically on Brazil because that was the only area they were entitled to by the Treaty of Tordesillas, Spain focused more on the tropics because they were more lucrative and they didn't have the resources or manpower to adequately administer the vast uncharted territories of North America, and Britain and France established colonies in the north because most colonial prospects in Latin America had already been snapped up by the Spanish and Portuguese.