In fact, many of the characters were considered throw aways-used simply to tell stories without thought of them becoming popular.
Did you know that Wolverine was originally slated to be a teenager and that his claws were in his gloves? At one point he was envisioned as being a real wolverine transformed into human form.
Wolverine, Thanos, Drax, Moon Knight, The Punisher, Morbius, Iron Fist, Hobgoblin, Sabretooth, Man-Thing, Starlord, Ghost Rider, Howard The Duck, Tigra, Red Wolf and dozens more started out a guest stars or try-outs in many of Marvel's anthology titles.
The Bronze Age was a period of change for Marvel Comics. Newsstand distribution was down, the Comics Code rules were relaxing and Marvel Comics was looking for new revenue sources to stay in business.
It was a time of experimentation, new media exploration and in some instances-desperation.
Readership was down as were publication numbers and much of the old guard was gone leaving Marvel with a second generation of creators-many of whom were comic book fans.
Time was money and many new creators, instead of creating new characters every issue, began to reuse many of the guest characters used previously as one-shots or try-outs.