In a word. Conflict.
There are differing viewpoints on this, but if you know the firm has funded a competitor or someone who might be perceived as a competitor, why pitch them given this conflict? It is unlikely they’ll fund you, and meeting with them may just provide insights into your business that benefit their existing portfolio company.
This does not imply that investors are looking to do this, but if they learn something useful from you about your business which may be relevant to an existing portfolio company, it’s not an unreasonable assumption that this info will make its way back to their company.
So why pitch an investor with a competitor in their portfolio? There are lots of fish out there. Search elsewhere.
Of course, investors with large portfolios may have a business that in some facet competes with you. Use your judgment but go in with your eyes open. If you are not comfortable disclosing something because it is proprietary or you worry about who might learn it, don’t share it. It may in some instances hurt your chances so this is a call that you need to make after considering the pros and cons of disclosing.