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!

What would you do if one of your sales reps called at 5 PM on a Friday, the last day of the month, on the final day of a bad quarter and said, "Good News - I closed ___________!"(insert any huge company here)

You'd get excited, your heart would beat a little faster, you'd feel relieved because things seem to be turning around, and you're thinking, "This is good, damn it."

You ask, "What did you close them for?"

Your salesperson responds, "They bought a ___________."
(the least expensive thing in the smallest possible quantity)

You know that  it would have been more appropriate for them to buy a _________________.
(the most expensive thing you sell)

You yell, "What?  Are you kidding? How did that happen?  And you're calling me to brag about it?  Are you crazy?"

The salesperson says, "But we're in!"

So here are some questions for you; 

Is it better to "be in" with something sold today, or to still be out, and hoping to sell them the appropriate solution tomorrow?

Is it better to say "they're a customer" now, or is it better to wait and do it the right way later?

Is it better to know you have to sell them one more time, or take a chance that you may never have the chance to sell them what they really need?

Let's explore the opposite scenario.

Let's say that you failed to close a company who decided against buying what you were trying to sell them.  Does it make any sense to return to them and offer to sell them what they are willing to buy?

These situations occur on a daily basis in most companies, (at least the companies with enough activity taking place) and everybody has a different preference as to how they should be handled.  What would you do?

Scenario 1 - happy to sell them something or wait to sell them the right solution?

Scenario 2 - return and try to sell what they're willing to buy or move on?

Here's another way of looking at it. In scenario 1, you waited to sell them the right solution and they wouldn't buy it.  So now you have moved to scenario 2...