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.
If you had actually used both, it would be clear why people use Telegram over Signal.
Signal doesn't even have a desktop client for any OS!