The Sims 1 is a much harder and different game because it originally only was meant as a small sidegame to the SimCity where the focus was a little more at the sims who lived in the houses than upon the city and its economy. So TS1 was targeted at the older and more experienced gamers who usually played the SimCity and TS1 had a low budget because EA didn’t really expect it to sell very well.
But to EA’s surprise especially very young teen girls loved TS1 even though they usually never had played the SimCity at all. Therefore TS1 got a lot of EPs which EA hadn’t planned at all because the sales numbers for TS1 and its EPs were huge and way above what EA even in their most optimistic moments would have expected.
For those reasons TS1 became a game which was quite hard because it was targeted at older gamers and with cheap primitive outdated graphics which made it look like something from the beginning of the 1980s and the EPs became quite difficult for EA to make and for us to install (it didn’t even have a chance to work unless we installed them in the right order and even if we did it often didn’t work in our first attempt).
So TS1 had a short lifespan of only 4 years while all later Sims games have had a lifespan of 5 yrs. But already TS2 got a much bigger budget and instead was targeted mainly at about 13 yrs old girls. Therefore all later Sims games have had much better graphics and a lower difficulty degree than TS1 had ;)