They did that by building boatloads of cheap shoddy phones (with the occasional exceptionally good model like the Galaxy models and some of the HTCs). Even in the UK we saw thousands of horrible handsets on sale (some still are) with Android 2.1, 2.2 and 2.3 like the ZTE models under other names, T-Mobile Pulse, Vodafone Smart, LG Optimus, San Francisco etc. all sold between £25 and £75 on PAYG.
Apples cheapest phone is £329 so volume has always favoured Android, but I think we might see a big threat to Android with the Windows Phones taking their share, Nokia have already started selling some excellent phones, WP8 is arguably better than Android and as the cheaper models start hitting the shelves it's going to take market share from the existing budget handsets (which are mostly Android now).
I really think Nokia have the ability to produce cheaper handsets that are built better than the equivalent Android phones and WP8 is more user friendly, Android is a bit of a nightmare for the casual user, current phones can be on 2.1, 2.2, 2.3, 2.3.5, 2.3.6, 4.0, 4.1 or 4.2 all with differences to their UI, the tile system on WP means everything the casual user needs is on the home screen, calls, texts, email, Facebook etc without the need for widgets or shortcuts.
If there is a OS in danger of a decline in the next 2-3 years, it's more likely to be Android than iOS.