Because until the BCS, there wasn't even a national championship game. So, everyone relied on season ending polls to determine the national champion. The AP poll was generally seen as the most credible, but there were somewhere between 10 and 20 different polls, and so on most years there was not a consensus national champion. 5 polls may say one team is the national champion, but a few other polls may have a different team. If just one out of the 20 some odd polls said you were the national champion, then the school could claim it. However, most schools with integrity, don't try to claim championships that were obviously given to them by an outlier poll, but technically it is an unclaimed championship.
Lets look at
1973 for example:
The UPI, AP, FWAA, and NFF were the most credible polls at the time. Alabama had the UPI on their side, while Notre Dame had the AP, FWAA, and NFF on theirs. Both Alabama and Notre Dame claimed that national championship. Michigan, Ohio State, and Oklahoma had polls declaring them national champions, but they did not claim those titles because the polls weren't as credible or they conceded that another team was better that year.
Alabama 11–1 Bear Bryant B(QPRS), UPI
Michigan 10–0–1 Bo Schembechler NCF, PS
Notre Dame 11–0 Ara Parseghian AP, BR, FN, FWAA, HAF, NCF, NFF
Ohio State 10–0–1 Woody Hayes NCF, PS, R(FACT), SR
Oklahoma 10–0–1 Barry Switzer CFRA, DeS, DuS, SR
Have fun
