Are suppliers afraid of something and if so, what?
Yes, they are afraid in a smart way.
Many of their inventory systems are running on IBM AS/400's or other big boxes (non-LAMP platforms), wired to 5250 terminal emulators, possibly via token ring. Their database is DB2, or perhaps Oracle. Internet access is the exception, rather than the rule.
There is the physical wiring (token ring versus Ethernet), differing networking protocols, authentication schemes, and database access protocol stacks to navigate.
And these existing systems are all live and running, so if you disrupt their workflow in any way (programming bug, etc), you'll be liable for thousands in downtime costs.
Their upside is minimal, maybe reduce headcount of people answering the phone, but add headcount of high-cost network administrators.
If you can make the numbers work, by all means you should invest all-in. You'll probably spend 2k man-hours getting your first customer up and running, and (assuming they pay all of your invoices), you may have a product that you can resell to other wholesalers running identical hardware and software.
The world is not entirely white box LAMP architecture yet. If it were, the problem-space is small, and the solutions are obvious.