Why is the government trying to exclude a key defense witness from testifying?

Last week we discussed what we called the "unindicted co-conspirators" in the Plantation cops mortgage fraud case, in essence our theory is that the banks that lent the money for what the government alleges were fraudulent real estate transactions were in on the deal.  In other words, the lenders knew about the inflated incomes and the bogus supporting documents on the loan applications, as a matter of fact it seems based on the documents that we've found in the court dockets that the defense has an expert witness that is willing to testify to the lenders complicity in these transactions!  While cruising the court docket I came across this motion...

daryl radziwon motion for expert witness

From the looks of it, it appears that a defense witness named Dr. Gary E. Lacefield, an expert in the area of mortgage fraud, is prepared to testify that the banks involved in these transactions were well aware of what was going on in order to obtain these loans and in fact may have helped encouraged some of the fraudulent behavior that led up to obtaining these loans!  Considering the potential magnitude of Dr Lacefield's testimony, why would the government look to exclude him from the trial?  If indeed Dr. Lacefield could prove that there was lender complicity in all of these deals, isn't the defense entitled to use him?  Check out Dr. Lacefields background here, considering his background, can you guys see any reason why he shouldn't be allowed to testify?