"Why did he and the others keep their Syrian citizenship?"

Are there any countries where people have the option of giving up their citizenship just because they no longer live there? Maybe there are, but my understanding is that the normal position is that that they don't have that option. (And that has been the basis on which disaffected citizens of countries such as the UK and the USA in times of conflict have been jailed and even executed.)

My brother was born in Argentina, and the family moved back to Europe when he was about three. Many years later he was thinking of paying a visit to out relatives out there, and he learned that he was liable for miitary service. He had to wait until he was middle-aged and no longer eligible to serve in the armed forces before he was able to pay a visit to the country he had been born in.