Full Idea
The central argument for foundationalism is simply that all other possible outcomes of the regress of justifications lead inexorably to scepticism.
Clarification
The 'regress' is every belief being endlessly justified by another belief
Gist of Idea
The main argument for foundationalism is that all other theories involve a regress leading to scepticism
Source
Laurence Bonjour (Externalist Theories of Empirical Knowledge [1980], §I)
Book Reference
'Epistemology: Internalism and Externalism', ed/tr. Kornblith,Hilary [Blackwell 2001], p.11
A Reaction
If you prefer coherence to foundations, you need the security of reason to assess the coherence (which seems to be an internal foundation!).