They're not small companies, the managed care organizations (MCOs) are some of the biggest companies in the US.
But I do not think their lobbying is what prevents single payer. For one, single payer (taxpayer funded healthcare) still uses managed care organizations in the US. The only difference is that instead of paying insurance premiums to MCOs, you pay taxes to the government. The government then goes out and hires the MCOs to ration healthcare and implement various formularies and double check doctors' orders to limit spending and prevent waste. This currently happens with Medicare (over 65 years of age), Medicaid (low income/wealth), and Tricare (for military).
So the MCOs are involved in the healthcare delivery no matter what. The only difference is for non military, non poor people pay MCOs insurance premiums instead of taxes to the government, with which the government then hires MCOs to do all the administrative work of dealing with patients, doctors, finding fraud, approving/denying medicines, negotiating pricing, etc. Although, the government sets pricing in big ways too, via reimbursements for Medicare/Medicaid.
In any case, the elephant in the room is and always has been how much do people with money want to pay for people without money. The more obfuscation there is, the less healthcare is delivered, and the less it costs people who would be net payers into the system.
Sidenote: I think governments (politicians) like the system of using MCOs because they take the heat for any rationing or mistakes in the implementation. It's not the government denying you, it's UHC denying payment, even though the government gave the formulary to UHC to implement.