Why do some plant species have lobed leaves, while similar species in the same habitat don't?
This is a question for which, I think at the moment, we don't have a clear answer. 

It is important to bear in mind that the leaf plays a number of important roles in the plant (photosynthesis, thermoregulation etc.) so leaf shapes probably evolved through a process of successive trade-offs. This may make it difficult to identify the exact selection processes operating on any one species. In contrast, something like the eye has a well-defined single function, which in principle at least, makes it easier to understand the link between form and function.

From Niklas (1988):


  Life history and optimisation theory suggest that the number of
  phenotypic solutions that allow for different equally successful trait
  combinations increases as the number of trade-offs increases – a
  conclusion that applies to traits within the leaf (e.g. for shape) as
  well as to leaf–branch relationships.


However, there are a  number of ideas to explain leaf shape diversity which include:


Thermoregulation


It has been shown that by adding lobes to leaves, the rate of heat transfer across a leaf is greater than that of an unlobed leaf of the same area (e.g. Gurevitch and Schuepp 1990). So, lobed leaves may be selected for under certain environmental conditions.


hydromechanical constraints


Lobed leaves may have greater hydraulic efficiency. For smaller veins, hydraulic pressure increases as they present an increased resistance to water flow. This places stress on the deliate outer leaf tissues. If  lobed leaves have relatively less mesophyll tissue than large, highly conductive veins, they may have reduced hydraulic resistance compared  unlobed leaves (Sack and Tyree 2005).


optimization for photosynthesis
adaptations against herbivorous animals
random (non-selected) variation. This is a possibility, although probably unlikely due to the importance of the leaf in plants.


So, some or all of the above possibilities (which are not necessarily mutually exclusive) may shape the leaves of species which may appear to have undergone very similar selective pressures. 

This is a good read by Nicotra et al. (2011) which summarizes the state of play.

Gurevitch J, Schuepp PH (1990) Boundary layer properties of highly dissected leaves – an investigation using an electrochemical fluid tunnel. Plant, Cell &amp; Environment 13, 783–792. doi:10.1111/j.1365- 3040.1990.tb01094.x

Nicotra et al., The evolution and functional significance of leaf shape
in the angiosperms, Functional Plant Biology, 2011, 38, 535–552

Niklas KJ (1988) The role of phyllotactic pattern as a developmental constraint on the interception of light by leaf surfaces. Evolution 42, 1–16. doi:10.2307/2409111

Sack L, Tyree MT (2005) Leaf hydraulics and its implications in plant structure and function. In ‘Vascular transport in plants.’ (Eds NM Holbrook, MA Zwieniecki) pp. 93–114. (Elsevier Academic Press: Burlington, MA, USA)