Short answer: Even though you're not "required" to change the oil at 50-hour intervals (25-hour intervals if you're using 100LL (AvGas) more than 30% of the time), it's a "best practice" that is pretty uniformly agreed upon by all of the Rotax "heavy maintenance" shops, based on their experience over many years.
Longer answer: Technically, the "official" guidance comes from Rotax Maintenance Manual Line. specifies that the periodic inspections must be performed at 25, 100, 200, and 600 hour intervals in accordance with the schedule published in that same section. The 25-hour check is a one-time check after the first 25 hours of operation for either a new or overhauled engine.
There is a note (under "Special hr. check") that explains the column for 50-hour checks is recommended by the manufacturer but not essential, with the exception of oil change when operating with leaded AvGas (100LL). All the action items listed in that 50-hour column are oil related: oil change, filter change, cut open and inspect old oil filter, and cleaning the oil reservoir if needed.
Beyond that "official" guidance from Rotax, pretty much every US-based expert at the heavy maintenance shops also recommends that you change the oil at 50-hour intervals if you're running 100% MoGas, or every 25 hours if you are using a mix of MoGas and AvGas (100LL). The folks I've talked to indicate that experience shows them that longer intervals tend to build up more lead deposits in the engine and (more critically) in the gearbox. Changing the oil at shorter intervals dramatically reduces the sludge build-up, thus improving the odds of making it to the scheduled intervals for gearbox maintenance. It's basically "cheap insurance", since oil and filter changes are far less expensive than gearbox overhauls...
The other issue is that the longer intervals recommended by Rotax are based on the usage of Aeroshell Sport Plus 4 engine oil. This oil was specially formulated with Rotax's input, and contains additives that do a much better job of keeping the lead particulates in suspension, so they don't settle out and create "sludge" in the gearbox and engine. Rotax says you can use other oils in the engine as long as they meet the specifications stated, but the "big shops" tell me that use of oils other than Aeroshell Sport Plus 4 will almost certainly leat to faster buildup of sludge, and thus shorten the gearbox maintenance intervals. One way to help avoid that extra bulidup is to change the oil more frequently. Thus the "50-hours on MoGas, 25-hours on 100LL" recommended oil change intervals from the shops that see these engines and gearboxes coming in every day...
