It is the most appropriate place for them to speak about their concerns, because they have a large audience that is willing to listen to them. The democratic convention is part of how Americans choose a direction for their country. They need to know what that is before they make that choice.
The Democratic National Convention is a good venue for Sanders and Clinton to share their beliefs and concerns with supporters. Sanders can use the DNC as a way to call for reforms to be made within the Democratic Party. Clinton can use the convention as a way to appeal to voters for the general election.
The Democratic National Convention is happening this week in Philadelphia. This is a forum specifically designed for politicians - such as headliners Bernie Sanders and Hillary Clinton - to share what their beliefs and concerns are about political events. I am very much looking forward to hearing what they have to say.
These conventions are there for the purpose of announcing the presidential candidate and for voicing the issues that are plaguing this country and offering up solutions on how to fix them. Clinton and Sanders have the perfect platform where all eyes are watching them to voice their opinions and concerns on what needs to be done to get this country back on track and how they plan to do it.