So this argument is just a technicality. Please understand a little more about the background:
There is basically one person writing the app[1], and given the company has just a few[2] people _volunteering_[3] for them, you cannot expect them to release a large amount of code across so many devices. They prioritized the highest volume first.
Open Whisper Systems primarily develops a strong encryption protocol (Moxie's efforts). If you didn't realize, this protocol was adopted by WhatsApp[4] and also Facebook Messenger[5]. So, the developers of those other applications needn't spend time/resources on the encryption, but can release Desktop clients for people like yourself to enjoy.
People who use Signal trust Moxie. People who dislike Signal _may_ care more about features than the security properties of the software (note, WhatsApp doesn't open-source their software[6], and Telegram instead bets people cannot break their encryption[7]).
Also, their app will supposedly run on any OS that Chrome runs. I'm sure that was the intention.
[1] https://github.com/WhisperSystems/Signal-Desktop/commits/mas...
[2] https://whispersystems.org/#team
[3] https://whispersystems.org/workworkwork/
[4] https://www.whatsapp.com/security/WhatsApp-Security-Whitepap...
[5] https://whispersystems.org/blog/facebook-messenger/
[6] https://www.whatsapp.com/opensource/
[7] https://telegram.org/blog/cryptocontest
