Bond market is perceived to have different per unit risk than the stock market.
The 90 day T-bill is called "risk free" because it has the full faith and credit of the United States Treasury. They are the only ones who can print money..Therefore no risk of loss to holders of that bill. There is no equivalent in the stock market, no risk free money except for periodic very short term arbitrage which is not available to the public.
For all other bond issues, risk is evaluated based on counterparty and credit worthiness, and so bonds receive a rating from credit rating agencies. Depending on this rating, they are able to be sold at various prices.
If one can obtain a risk free premium of say 8%-10% by purchasing a bond (duration is another subject) they may elect to do so, instead of taking the risk of losing money in the stock market. If on the other hand the interest they can obtain is only 5% (as it is now) and they need the money, they may be motivated to take that additional risk in the stock market. Both markets (equity and debt) compete for available capital.
Are we clear?
Steve