Agree with Xover's point.
Also, while I dearly love Morrowind, technology has advanced a lot in the past 10 years and its outdated graphics and less sophisticated combat will almost certainly fail to impress many gamers. The same applies to Oblivion to a lesser extent.
The only people I can imagine wanting to play them are non-TES fans who buy ESO, really get into its lore in a big and want to visit the earlier games to experience more of that first-hand. Those games can be picked up quite cheaply from various places (eg. Steam).
Could also have a negative effect PR wise. ESO looks to be pretty amazing but if ZO tried to bundle it up with some of the older single-player titles, that could be viewed by some potential customers as ESO not being a strong enough product to sell well on its own. I.e. it could be perceived as desperation rather than generosity.