Open source software/frameworks work good individually, but it takes some effort/time to integrate them, the main challenge is the interoperability issues between the different frameworks. This is where companies like Cloudera, Hortonworks, MapR and others come into play. They take the different frameworks from the Apache Software Foundation and make sure they play nice to each other. Not only do they address the interoperability issues, but also make performance/usability enhancements.
Apache Bigtop takes this effort to a community level from a individual company level. Bigtop can be compared to Fedora, while Cloudera (CDH) / Hortonworks (HDP) / MapR (M5/M5/M7) can be compared to RHEL. Red Hat provides commercial support for RHEL, while Cloudera / Hortonworks / MapR provide commercial support for their own distributions of Hadoop. Also, Fedora has some of the leading edge and more variety of softwares and same is the case with Bigtop also. A lot of Apache frameworks (like Mahout / Hama) are included in Bigtop, but not in the commercial distributions like CDH/HDP/M*.