Because some managers and Funds are obligated to own Bonds and Fixed income products by their investors.  Like 10% of their portfolio for example must be allocated to Bonds.  That in recent history has been just burning money but they are obligated to own it . Now MSTR offers them something (convertible bond offering) in that space that actually performs and managers will buy up every offering that they possible can. 

 I think we will see open defiance of such rules in the next 5 years. Then we shall learn what the enforcement mechanisms are.