For the reason Rais notes, i.e. the "sales pitch" provided by private WAN vendors. However, is it true?
Maybe, maybe not. I've used both, side-by-side, in a fortune 100 company for both North America and the rest of America's links. Generally, Internet VPN performed very, very well - much, much less expensive than private WAN bandwidth. (Also in many parts of the world, much easier/faster to obtain too.) Seldom had a congestion issue except at edge connections (the latter also true with MPLS). The "secret" with Internet VPN is to not share that link with "raw" Internet traffic, i.e. restrict it to only Internet VPN. (NB: If you need "raw" Internet, obtain a separate Internet connection for it.) With Internet connections dedicated to just VPN, you can manage bandwidth "on the other side" (NB: also, generally, has more QoS capabilities than provided by MPLS vendors).
Notwithstanding the foregoing, one approach some companies use is to use MPLS for "business critical" traffic (e.g. like VoIP or critical apps) and Internet VPN for non-critical taffic (e.g. like data backups or [server-to-server] email, etc.).
