There's lot of mod available for Bitmain but can't find any for other manufacturer. Why ?
Because most mining gears out there are made by Bitmain, I believe there are at least 8 Bitmain Antminers for every Avalon and Whatsminer out there, so the custom firmware devs try to target the majority of miners which is pretty logical.
The second reason is what NFW states above, the only reason I use custom firmware is that it does fix some broken miners and gives me the basic tweaks I need that the stock doesn't have, but the other miners have at least the base settings which reduces the need for any custom firmware that has fees on it, honestly, if Bitmain slightly improves their firmware the custom firmware market will take a big hit, but alas, they seem like they are happy with how things are.
On a side but related note, I know Vnish team is working on MicroBT firmware, I have no clue about the exact process, but when any dev tells you "soon", it could be 4 years.