Campaign finance law means that there are separate pots of money for before and after the nomination at the convention
The very wealthy favor Republicans, which means they tend to have more money in their general election account
Having less time between convention and election day means that the dollars-per-day you can spend out of the general election funding is greater
This pushes Democrats to try and hold their convention as late as actually possible in order to enable them to match the Republican spend rate during the final weeks of campaigning