The simple answer is that there was strong competition from other European powers, including the Dutch, the English, and the French. The early explorations tended to be concentrated geographically, and use sea lanes favorable to each nation.

When Europeans discovered the Americas, Spain had a lead but it wasn't a very big lead. This map is an excellent resource showing the exploration routes and dates of various expeditions.

You can see clearly that Spain's lead was very short, only a very few years, before other powers started their own campaigns to establish outposts in the Americas. To be clear, the Spanish did try to expand farther into North America, but these attempts were beset by challenges. To quote a previous answer, in 1573 the Spanish changed the laws on exploration dramatically, both by limiting licenses for exploration and focusing more on missionary work. Secondly by that point they had discovered great riches in today's Mexico, Peru, etc. and they knew they they wanted a frontier to stop other European powers from expanding southwards.