To give a more precise reasoning as to why 1.7.10 was never worked on:
At the time that Sponge started, Forge had not had support for vanilla clients on forge servers with forge mods that did NOT require a client mod to be installed (in which case, SpongeForge servers would HAVE to have at least forge installed) with Minecraft 1.7.10. In 1.8.0, Forge added vanilla client support, and that was the first version Sponge API was being built upon. Alongside the structural changes in Minecraft’s implementation that SpongeAPI was being designed around (some concepts such as BlockStates were only introduced in 1.8) could ONLY have been done with 1.8 and above.
Likewise, with the constant progression of development with Minecraft itself, we could not feasibly maintain newer features being added/changed to SpongeAPI when the project had just started off.
At this point, with all of the advances that we’ve made with Sponge’s API and implementation, it’s not feasible to "back port’ any of of the changes to 1.7.10, and maintaining such a version would drain the already limited time of the team.