I realised that it has a big demerit that the URL it gives does not contain the content of title of thread.
It isn't a demerit as far as I know because it's better in that way for many people. We don't need the title of a thread in a URL as that's not necessary at all, and trust me no one reads the title of a URL that much these days.
Was this not taken into consideration while choosing forum software ?
This forum is based on an old version of SMF and it hasn't been upgraded in years and it may never get any update ever. The new versions of the SMF will change the whole look of the forum and many members are against such changes. I enjoy this forum the way it is because it's one the best looking forums on the internet, and I don't think that the forums needs an update just to have tile or contents of a thread on the URLs or for any other purpose.
Are there some special benefits of SMF over other popular forum softwares like xenforo or vbulletin?
Yes, there are many benefits for SMF over other popular forum software. The main benefit of SMF is its simplicity and ease of use. You can load SMF based forum on a low-end device without any lags or delays and I believe it adds less burden on the host that's hosting the SMF on their system. Mostly the forums with low traffic are hosted on cheap VPS servers and SMF works like a charm for such forums.