I'll be the first to admit that selling to big companies can take much longer, may include many starts and stops, musical chairs, committees, task forces, layers of management and additional competition.  But beyond those considerable annoyances, what makes it so difficult?

I'll offer my thoughts and you can feel free to add your own:

1. Staying Power - some salespeople become frustrated by these opportunities tending to either not gain traction early on, or stall after gaining traction.  They become impatient and simply do not pursue them.
2. Royalty - some salespeople become intimidated by the additional zeros that could be part of a possible deal and treat the opportunity quite differently from their regular opportunities.  Instead of asking tough questions, pushing back and challenging their prospects they become facilitators, failing to differentiate themselves from their competition.
3. Decision Making - In large companies it can be difficult to figure out who is actually making the decisions and even more difficult to meet that person.  There must be a compelling reason for the decision maker(s) to get involved early in the process and failure to ask tough questions means it may not be very compelling for them at all.
4. Money is no Object - For some reason, salespeople tend to believe that big means unlimited funds.  As a result they invest an awful lot of time pursuing opportunities that they never completely qualified.  Being big does not mean they will spend whatever you charge them and, in many cases, they expect to pay less, not more than their smaller cousins in business.  Some may even expect you to conduct tests and trials at no cost to them.
5. Happy Ears - Because salespeople are hearing signs of interest, they may pursue big opportunities for much longer than they should.  Slight interest should tell the salesperson to turn around - dead end - but instead seems to invite them to play it out.
6. Relationships - Most salespeople fail to develop strong relationships with all the various players and as a result, may have a champion, but fail to have an entire team on board with their solution.
7. Chauffeurs - I've never been much of a believer in the Champion or Influencer goal.  Instead, I believe that salespeople must identify a Chauffeur - someone who can drive you to the person who cares and has enough power to do something - like make a change or pull the trigger.
8. Fit - Just because a salesperson is trying to sell the big company, does not mean that her solution actually fits the big company's needs.  Back to thoroughly qualifying; will your size, location, track record, capacity, team, references, timeliness, delivery, availability, expertise, experience, pricing model, quality, country of origin, contract, compatibility, ownership, terms, or billing/payment method be issues?
9. The salesperson - Sometimes, a salesperson who does fine with smaller companies, just does not have the self-presentation - poise, posture, wardrobe, polish, look, vocabulary, accent, hygiene or eye contact to take it up a notch and present in the board room.
10. This is where you can contribute - simply enter your comment for what you believe is the 10th reason and I'll choose a winner!