Because the dominant ISP (Telenet) has enabled IPv6 by default on all their residential gateways. And because they (possibly somewhat surprisingly) have some smart people working for them.
That's remarkable, since Telenet is part of Liberty Global, who owns so many ISP's. Eg. neighbouring country The Netherlands has Ziggo (also owned by Liberty Global). Adaptation in The Netherlands is only 9 pct. Makes me wonder if Belgium is used as a technical testing playground for Liberty Global or if it's just a smart move by Telenet.
Depending on where you are in the Netherlands Ziggo will give proper Dual Stack or DS-lite to new customers. It reflects the networks that Ziggo was cobbled together from. If you are in a former UPC area you probably want to call them and ask them to downgrade you back to IPv4 (they do this for free). Their DS-lite solution employs carrier grade NAT which blows dog chunks.
DS-Lite[0] means you have proper native IPv6 but only a tunnel on top of that for v4, so it's unlikely that IPv6 connections are negatively affected by this setup.