why we have different pages for different versions

Because they are completely different games. Look at other series (e.g Sonic) and you'll see that handheld games are separated from console versions. Why? Because they're completely different and so deserve their own page.

It's not just handhelds either. Using the Sonic series again as an example, they have Sonic Unleashed PS360 and then Unleashed Wii/PS2. These versions work differently and so deserve separate pages.

If we mixed all the different versions into one page, it would turn into a bigger mess than just listing all the different versions on the Series page. I know there is a drop-down menu to look at separate versions, but there's also the problem of differing categories specific to a game (e.g. HP1PC has Superjump%. If we put Playstation with this, it would have an empty, unneeded Superjump% category) as well as differing rules for the same category (e.g. HP2 PS1 has all cards, Flipendo Duo, all secrets, all Gold Wings, whereas HP2 PC has just all cards and all secrets). It would be a mess to get that all sorted out. Plus, ILs could be messy with categories, but I suppose for just the ILs themselves we could do this sort of thing " PC: Gryffindor " "PS1: Lesson Races ".