So what are the incentives for run a public electrum server? Help others offering the server for propagate transactions for free may not be an incentive at all.
Some want incentives and the only one I can know of is linking addresses together and maybe also to a list of IP. Maybe this information can be sold to chain analysis companies although I expect they have a fair share of SPV wallet servers themselves.
Some don't care about incentives and simply want to give back a little. Maybe had gains from Bitcoin, maybe they are happy to help Electrum network... In the same way many developers did work on tools without asking for incentives, others simply help the network without asking for incentives.