Just wondered this as ive come across this issue many times. Id rather not have several vs versions, but the fact that i cant open 50% of the sourcecode i find is rather annoying. I really hope there is some way to make my 2015 community versions able to open these older solutions(most often 2010/2012). 
Cheers!
When you say ‘Can’t Open’ what do you mean exactly? when I open solutions, I am often asked to ‘Upgrade’ the solution and they work usually… what messages are you actually seeing? it may help if you post screen grabs of all relevant information so perhaps people can help better…
Maybe those projects have dependencies such as say some third party library that is not in NUGET or something that you need to include manually?
We will be throwing the entire Niagara Falls at you by guessing, so some factual data would help narrow your issues down…
Valentine.
Usually VS says something li9ke this "this version of visual studio is unable to open the following projects " and the files in the explorer will be marked with (incompatible). Hope this helps
Take a screen grab [Snipping tool does this well for most situations] and the other side of the process, what are these projects you are trying to open, and they .sln files? where did you acquire them from, can we reproduce the scenarios?
The more you provide, the more we can assist, obviously we understand you may not be able to know where to begin other than the issue at hand, as we all have that issue too so I hope my line of inquiry helps…
Its some sln files from an older XNA Pathfinding project im trying to open. Its from the sourcecode library on this link https://github.com/DDReaper/XNAGameStudio. I realise XNA is what Monogame used to be? I guess. But shouldnt me having Monogame installed be enough or are the something in the older XNA version that i need to open these solutions?
For xna4 you need VS 2011, I’m pretty sure there is no way around that.
I would presume you required XNA Studio, if you want to use MonoGame I presume you could just copy the code from the .cs files manually, I could be wrong but logically I would assume this would be the route…
Gonna try get VS 2011 and give it another shot, opening the files one by one to read the code but not be able to run it is… Less than ideal
Well, when you port it to MonoGame, you will still be doing the exact same thing whether within VS or not 
but I know what you mean
Even if you cant open the project, you can still open the individual class files in a NEW project in your current editor… Then all it takes is few updates to the using statements at the top, drag/drop all the content over, make sure the new name_space is enforced everywhere, and you can run it…
I have done that with small projects that wouldn’t open for whatever reason. Maybe it will work for you too.?
I’ll update the homepage with some getting started info @Kasper_Kasper
But as shown in the intro videos on my MG YT Channel you can open ALL of the XNA projects in VS2015 using the awesome MXA project. This enables XNA support in VS using the XNA redistributables.
You can find the MXA project here: http://mxa.codeplex.com/
The long term plan with the XNAGameStudio site is to convert the projects to source in MG. A long road but will get there.
Hope this helps