Diversification
This relates to the old adage "don't put all your eggs in one basket". While simple as this may sound, underlying it is a fundamental consideration in investing. That is the issue of investment uncertainty or risk.Just as one cannot predict with 100% certainty the outcome of a horse race, one cannot predict with certainty if a bond issuer will or will not default on payments. That is why it is unwise to keep a majority, much less all, of one's money in a single investment issue.
In the recent past there have been tragic examples of employees whose 401K accounts were wiped out because they had all their money in their employer's stock at the time their employer collapsed. A notable example is Enron.
Diversity is the key to reducing risk in any portfolio, including bonds. By diversifying within a portfolio, your overall investment is less volatile and less likely to be affected by the performance of one single investment.