The makers may have thought that just like Mughal-E-Azam (which had re-released a few years earlier in a coloured version) or Sholay (which has been re-released a number of times already), even Hum Dono would manage to find an audience for itself. However, when compared to Mughal-E-Azam which had seen good success coming it's way on it's re-release 6 years ago, Hum Dono Rangeen didn't find any takers.
There are three main reasons for that. First and foremost, there was hardly any promotion due to which audience was unaware if Hum Dono Rangeen had actually released in theatres. Yes, Dev Anand did host a premiere for the film and also reached out to the public via a few interviews and appearances etc. However, in the times of an aggressive in-your-face promotion, this wasn't good enough.
Secondly, Hum Dono was not a classic but just a good film. It was loved when released; it had also done well at the box office. However, it didn't have the kind of shelf life as the likes of Mughal-E-Azam or Sholay and wasn't enjoying multiple re-runs on satellite or via home video.
Thirdly, and most importantly, it is not a big screen extravaganza a la Mughal-E-Azam or Sholay that would have brought audience in dozens to theatres. Both films were hardcore entertainers and had number of factors (action sequences, romance, memorable dialogues etc.) that were enough to entice audience to check these out again. Hum Dono too had hit songs and powerful performances as its selling points. However, number of reasons as stated above meant that audience were not inclined enough to re-live the dream that Dev saab carried.