It's because we almost always use words like Tatws, Moron(carrots) and coed(trees) in the plural and also since the base form is the plural (they're called singulative plurals). If you want to make them singular the ending is either changed or you add on an "-en" or "-yn" ending I. E Tatws becomes Taten, Moron becomes Moronen and Coed becomes Coeden (an example ple of the "-yn" ending is Plant(children) which becomes Plentyn. Teganau isn't one of these and the singular is Tegan.
As is explained before, the plural form of 'tatws' and 'moron' (potatoes and carrots) are more normally used.
In addition these initial units were modeled on the 'Welsh for the family' course and as such the patterns of vocabulary and grammar introduced are the ones that parents would find most useful in daily life.
Thanks to both of you; now I can see why it makes sense to introduce the plural for those nouns first.
For tegan I would suggest introducing the base (in this case singular) form before the plural as well. As it is (2016-06-24), tegannau is introduced in Wanting1-3, tegan in Wanting1-4. Or does that order of introduction follow the one in 'Welsh for the Family'?