Well you can also say that because Windows Phone is not popular in certain regions, developers may not see the incentive to support them. One major issue is language. For example, if I released an app into China but does not provide Chinese translation for the app, my app may get rated down by Chinese consumers because of that. However I don't see a reason for me to fork out money and time to get someone to translate the app to Chinese and to maintain the translation unless I believe there is a huge market for my app in China. Another issue is restricted content for certain countries. If my app contains certain content that may be considered unsuitable in these regions, even if I have selected them, it's not going to appear for those regions. A developer may be even afraid that his / her app may even fail submission because of the content which will cause them time and money to resubmit again. Certain regions like Brazil may also require additional certifications by rating boards on the content before you can submit to these markets. Again the developer would have to spend more time and money. The best advice that I can give is to let the developers know that their apps that you love is not available in your region. Give them a chance to know that people from your region are interested in their apps so they just may take that extra mile to support you.
Remember this is a developer issue, not a Windows Phone Marketplace problem. If I'm not mistaken you can sell apps to more countries than Google Play. And you only have a problem with paid apps; there is no reason why you cannot download and use free and trial apps from any region, just that the app is not shown in your local marketplace. Switch the region, restart your phone and you can download them. You only need to switch back to original region if you wish to purchase apps. So there shouldn't be any reason why you cannot use the official free Twitter app. It's only paid third-party Twitter apps that you will have a problem with.
I'm also tired of some people telling others that they went with Android instead of Windows Phone because of this issue. This is not really true, as in my region you are not able to purchase Android apps as well. The reason why they can get to use those apps on Android and not Windows Phone is because they are using cracked versions. You are free to go that route if you wish but this is one reason as a developer I choose to support Windows Phone instead of Android. Windows Phone would possibly also be more popular in certain regions when people there stop telling lies and convince potential buyers from buying them. A non-tech person I know sold his feature phone and wanted to get a smartphone. He saw my new Lumia 620, I let him use it for a while and he liked it. He went to a shop that supposedly sell Nokia phones but seems to have more Android phones on display. He asked if they have the 620 and the shopkeeper said yes but when asked whether he can have a look at it, the shopkeeper said no, unless there is a commitment to buy the phone on the spot. Then the shopkeeper tried to convince him to buy an Android phone instead. When he told me about this, I replied that's why I ordered mine online. So I'm always a click away from blowing my top when I hear someone tells me my phone is lousy because if my phone is any good why no one is buying it here. WELL PEOPLE MAY BUY IT IF THEY WERE ALLOWED THE CHOICE!!! Sorry, had to let off some steam