That's because I think you will find that owncloud is web server based, so when you installed it you installed another web server and owncloud set this up to be the working web server on your pi.
So owncloud has replace your previously installed web-server (Apache2) so now when you connect to your pi's IP on the default port 80 it's the owncloud web server that responds.
As both web servers are set to listen on port 80 by default the last one installed will be the one to respond, if you want to run 2 web servers on you pi you will need to change one of them to listen on a different port and then when connecting to the pi's IP you will need to specify which port you want to connect to.
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