my first real SLR that I bought was the 450D, so before the age of digital how did Canon differentiate its model line up feature wise? I mean back say year 1998 the determiner of IQ was the lens and film and the paper that it was printed on. My point is, why have so many sensors across a brand's line up, with 1DX sensor, 5D3, 5D2 and then the rebel line up of sensors, if all camera's just over a decade ago had the same ability to capture light correct? So what you pay for then is build quality, FPS, and metering etc.
In short: Different equipment price points to meet different consumer demands.
It is no mystery that larger sensors, sensors with the greatest performance specifications, and bodies providing all the features any photographer could need/want also costs the most to produce, and are not needed by every user. Digital photography technology has allowed manufacturers to actually tailor the operation and output of the cameras to meet different user demands and budgets.
At the lowest price point, Rebels for Canon, the manufacturer focuses sensor design and implementation more on the criteria of needed output (jpg) - even if it impacts the cameras base (raw) performance. Features will generally be only those necessary to provide functionality and competitive marketing (albeit gimmicky features sometimes). This is not to covertly produce an inferior product, but because the typical purchaser at this price point will not need or desire post processing to get the results they are looking for.
In the middle price tier (Enthusiasts, and Semi-Pro) where the output demand can go either way, the manufacturer will focus sensor design and internal processing to provide the best aspects of both types of output. In addition features will be greatly expanded providing full control over the camera operation, and with reasonable accessibility such as dedicated buttons instead of diving into menus.
At the top tier you will typically find almost all features provided, and implemented in such a way that use of the cameras menu system may not be needed at all other than relatively permanent, optional, changes in camera operation. In this tier you will typically also find the best unhindered raw performance due to this sectors reliance on post processing - mainly because the images are turned over to production/marketing departments to be culled and processed - although jpg output is not worthless by any means as some professionals are satisfied by that output.
To continue with the film camera analogies; Think of it as if film cameras were made in such a way that they could only use one brand and/or speed of film, and only had certain features dependent on what film version you purchased. That certainly would not satisfy the needs of every photographer and there would have been a need for several versions of every model to appeal to different users. (Fortunately that was not the case..