You paid extra for the large, experienced corporation to do it. If it failed, it was outside of your power since they have a reputation and resources, but it just did not work out.
Startups have no reputation. If they fail, you'll be asked why you picked them, and any answer you give is based on your judgment of their abilities, which was wrong since they failed. If you say it was to get X done cheaper, you're the cheapskate who wasted X$ trying to save Y$.
There are also very often situations where your corpo has used corpo X for 5 prior/current projects and is satisfied, so picking them also gives you the backing of those other projects, should something go wrong.
It partially makes sense, but it's also largely office politics. You really don't want to have a complex explanation when you're in the crosshairs of some angry bossman 2 tiers above your boss.