It has been designed with this restriction
From the user point of view, you enter your DeLorean, set the desired datetime in a display, and it moves you there. Easy enough, right? But in order to do that, behind the curtain the time machine will need to know the current datetime and location, determine the destination location (by default it will be the place where the "things" around you would have been at that time, a fair end-user expectation but computationally non-trivial), convert those human coordinates into the actual units with which it will work, map a route in the spacetime from the current location to the final one, and determine the needed motor impulses to "jump" there.
A time machine with a quite limited temporal range will be easier to build than one with a larger one. It's not just that you will need to load a larger spacetime map on its memory (which someone must have somehow calculated before!), but it will also need much more extensively testing (don't let me started with the endless number of regulations they must follow!).
At the same time, very few users will need to jump back more than 50-100 years, so it makes sense to segmentate the market with different products that differ in their available range. You build many low-range time machines that are cheaper to produce, and a few extremely expensive ones for those (typically companies) that need going that extra
mile century. If the average consumer would want to go on holidays to an out-of-range date they would simply use a commercial flight to travel then.
Thus we will be dealing with many time machines in the market with a limited time range (with slight differences e.g. a commodity time machine from a luxury brand may allow a time-span 5 years larger than their competitors) in which they may be used (according to the manufacturer).
Of course, the fact that the manual says "do not use outside the tested years" doesn't mean that the users wold do that, no matter it being extremely dangerous to the occupants. "Family dead in Mercedes-Benz time machine accident" is very bad for PR, no matter if they went out of their way to use it way outside the operational specifications. Plus government regulations.
So the manufacturer build the time machines to cover a time-range (let's say, fifty years) plus a safety margin, then lock it so you can't use it to jump outside the stated range. That's why it doesn't matter that you are already in the past when you try to go earlier.
A shady mechanical may be willing to remove that limiter from your time machine, but operating it more than a few more extra years would require a full revamp of the motor and its associated computer system. Obviously, that is completely unsupported, although military time machines may allow to bypass the limiter. There are some known cases where a low-range model of time machine is actually the same as an higher model but with a different limiter. These are popular among tuners, and relatively safe once they are loaded the pirated maps from the higher model. However, in some cases they would be tagged as the lower model precisely because in QA they failed to operate properly for a bigger range.