Why are "Clovis people" believed to first enter North America?

In the 1930s, archaeologists discovered spear points in the New Mexico town of Clovis, dated from around 13,000 years ago. The individuals who made them have since become known as the "Clovis people" and are believed by many to have been the first people to enter North America.
The "Clovis first" theory proposes that around 14, 000 years ago people traveled across a land bridge that existed between Siberia and Alaska. Once in North America, their journey took them through a corridor that opened up between the ice sheets in Western Canada making them the first to be able to enter the interior of the continent. It’s an elegant and almost biblical explanation, but more recent fossil finds question the theory. Some researchers believe the first people entered North America much earlier—perhaps as long as 20, 000 - 30,000 years ago.
In more recent years, the Pacific coast has been seen as an alternative route of entry—possibly at an earlier date than the Clovis. There is little direct evidence to support this, but fossil evidence from the islands of south-east Alaska tells us that this region, or part of it, was free of ice and may have been a refuge for animals throughout the ice age.
If animals such as bears and foxes lived here then why not people? Fossilized human remains have been found on Alaska’s Prince of Wales Island and much further south on the Channel Islands off California. These are thought to be as old as some of the Clovis finds and it may only be a matter of time before older remains are discovered.
If people did travel down the coasts between islands, they must have used boats. Although the coastal route still lacks a lot of hard evidence, the relatively recent discovery of a stone tool off the coast of British Columbia has added support to the idea.