Changing trams is easy, doesn't mean people want to do it! Direct journeys are always better than changing. Traffic can occur anywhere and while it might not be ideal, you have to weigh out the benefits.
They are across the road but but it's still extra walking. Especially if you are changing between buses. The proper interchange isn't there.
As for buses to Sheffield, they will have the market share over the tram based on the price difference and the minimal journey time difference. By the time you have got into Rotherham (however you got there), walked to the tram, waited and got into Sheffield, you have saved little-no time compared to just using the bus. With Stagecoachs biggest routes into Rotherham duplicated on the core sections by First, do you pay £4.90 and walk between the bus and train stations or pay the £5 and have an almost seamless interchange.
Supertram is too intigrated in my opinion. It should be on a separate website with only the basic journey planning stuff included on the Stagecoach site. The only way to view any useful information is by setting your location on the website (not useful). To view fares you have to use 'plan your journey', Timetables can only be seen trip by trip, not one combined timetable.
Compare finding information on Supertram to finding information on Nottingham Express Transit which I would argue is the closest match. There is a huge difference and ease of accessing information will be one of the reasons for Supertram having reducing passenger numbers for the past few years. Even in the 2018/19 fiscal year when the tram train opened, passenger numbers were down 400,000 passengers on 17/18.