Why does the FAA SPL require different endorsements depending on cruise speed?
When looking at the rules on ecfr.gov you will find references at the bottom of the paragraph indicating the Amendment, Federal Register notices, and other documents related to the rule.  

In this case it leads to the following FAA doc.  That document includes the following explanation for the rule;


  II.B.5. REQUIRE 1 HOUR OF FLIGHT TRAINING ON THE CONTROL AND
  MANEUVERING OF AN AIRPLANE SOLELY BY REFERENCE TO INSTRUMENTS FOR
  STUDENT PILOTS SEEKING A SPORT PILOT CERTIFICATE TO OPERATE AN
  AIRPLANE WITH A V Current regulations require student pilots seeking a
  sport pilot certificate to receive and log flight training in the
  control and maneuvering of an aircraft solely by reference to flight
  instruments. This training must be received before conducting a solo
  cross-country flight or any flight greater than 25 nautical miles from
  the airport from where the flight originated. It also must be received
  prior to making a solo flight and landing at any location other than
  the airport of origination. These requirements are detailed in § 61.93
  and are applicable to persons seeking a student pilot certificate to
  operate any category and class of aircraft. That section, however,
  does not specify any minimum flight training time to meet these
  requirements. In addition, current regulations for the issuance of a
  sport pilot certificate do not require an applicant to receive flight
  training on the control and maneuvering of any aircraft solely by
  reference to instruments. The FAA is concerned that persons exercising
  student or sport pilot privileges in airplanes with a maximum airspeed
  in level flight with maximum continuous power (VH) greater than 87
  knots calibrated airspeed (CAS) may inadvertently encounter conditions
  less than those specified for VFR operations due to their greater
  speed and range. Operators of these aircraft are more likely to
  encounter instrument meteorological conditions than operators of other
  categories of aircraft. In order to enhance the ability of these
  pilots to appropriately react to the possibility of encountering
  instrument meteorological conditions and the potential consequences of
  attempting continued visual flight rule (VFR) flight in instrument
  meteorological conditions, the FAA is proposing to require persons
  operating an airplane with a VH greater than 87 knots CAS to receive
  and log 1 hour of flight training on the control and maneuvering of an
  aircraft solely by reference to instruments. The FAA recognizes that
  persons may currently be authorized to operate aircraft with a VH
  greater than 87 knots CAS. To provide those persons with a reasonable
  period of time to obtain this training, the agency is proposing that
  the training be completed by 1 year after the effective date of the
  final rule. This training would include straight and level flight,
  climbs and descents, turns to a heading, and recovery from unusual
  flight attitudes. Due to the slower speeds and limited capabilities of
  categories and classes of aircraft other than airplanes, the FAA is
  not proposing that this requirement be extended to operators of those
  categories and classes of aircraft and airplanes with a VH less than
  or equal to 87 knots CAS. The FAA notes that for training to be
  conducted solely by reference to instruments in visual meteorological
  conditions, it must be conducted with a view-limiting device.


My take is that earlier rules covered the conditions with VH less than 87 KCAS.  The FAA felt it was necessary to add rules to cover flight at the higher airspeeds.