It really boils down to lender risk and how they can mitigate it.
There are so many moving parts to commercial loans (property value, property maintenance, changing market conditions, occupancy issues, rent, etc.) that lenders "usually" want to re-evaluate a property every so often.
By issuing notes that balloon in 1-5 years, lenders can re-underwrite a file to either refinance the property or call the note and make the borrower go elsewhere.
There is also a smaller issue that lenders like to diversify their portfolios. What is good today, may not be good down the road.
For example, hotels are now considered a bad bet and are very hard to finance due to falling values and high vacancies, but 5 years ago, they were in much better shape.
So, most lenders are trying to get most hotel loans off of their books. If lenders issued 10-30 year notes, they would be swimming in toxic paper. Well, more toxic paper.
Make sense?
NOTE: As Bryan eluded to, there are long term loans out there, but those are almost always loans that have a government guarantee (SBA, HUD, Fannie/Freddie, and USDA). That guarantee makes it possible for the lender to put their money out there for 20-40 years.
Some REITs, Insurance companies, and pension funds will also do long term loans, but those loans are for very experienced borrowers that usually have a portfolio of performing properties.