There are a few points to consider here.
First is purely mathematical if you have say 2 rotors with 26 possible outputs each you get 676 possible results, replacing them with two 36 output rotors gives you 1,296 possible results, adding an extra rotor instead gives you 17,576 possible results obviously much harder to crack.
The second point is almost all messages will use standard letters so this makes cracking the code much easier since the British can eliminate all rotor settings that would require a non-standard input.
The third point is ease of use, the entire point of the enigma system was not that it was unbreakable but that it was easy to use both encoding and decoding, while breaking would take so long that the message would be useless. It is easy to show that without electrical computing machines, enormous numbers of staff and any cribs decoding would take years (of course the British had all those things and possibly the brightest collection of minds ever under one roof). Adding extra letters would increase the likelihood of operators making mistakes, having to repeat messages or even giving up and sending urgent information in clear.
If you want to make the Enigma machine more difficult to crack there is a much easier way.
The designer arranged it so that no letter is ever encoded as its self, that is E never comes out as E, apparently this was supposed to make it harder to break but in fact this simplifies the problem massively. In addition to reducing the possible number of combinations it was easy to test possible cribs by checking to see if any letter matches if it does then you have the wrong crib, try it on a different message.
With out this quick and dirty way of reducing the possible messages that might match a crib decoding would be much harder and slower.