1- The Tabligh Jamaat has selected only six points. They have toiled hard behind their implementation . If you do perceive any deficiency in these points, you may increase them and strive accordingly. Other than this, all we can say is that this is a recurrent question.
2- One of the reasons for selecting the traditional six points is that they act as primary fundamentals in spiritual development. Practising upon them simplifies implementing the remaining branches of deen. Experience bears evidence to the fact that whomsoever worked hard on these six points was ultimately rewarded with religious enhancement in accordance to the degree of his endeavour. You are quite aware of the fact that a Non-Muslim, immediately upon accepting Islam, is not advised to observe Salaah (prayer) and sawm (fasting) etc, but at the outset is invited towards the Shahadah/Kalima. Only then is he instructed to observe Salaah and sawm. In the same way, a Muslim who, owing to his prolonged neglect towards Islam, develops some deficiency in his I'tiqadaat ( tenets) and Ibaadaats (worship), at the very outset he is invited towards these six points. Once he is firmly grounded in them he may return to the other issues of deen. Many incidents bear testimony to this fact. Following the persistent efforts in this path, many people, amongst other things, finally gave up consuming wine, became regular with their salaah and other devotions and they developed respect for the Ulama and love for the past shuyukh. But when did this come about? Only when they aptly toiled behind these six points. Hence it would be correct to say that striving at these principles sows within the heart seeds of the magnanimity and love for deen and the seeds of Imaan (faith) and Yaqeen (belief). The correct nurturing of this seed provides great hope for the tree of Islam blooming in the future.
3- Another point to note is that not a single Jamaat nor Islamic faction has any objection against any of these six points. The first point is the Shahadah/Kalima, the second is salah, the third is Ilm & Dhikr, the fourth is Ikraame Muslim (honouring the Muslims), the fifth is Ikhlaas-e-Niyyah ( sincerity of intention) and the sixth is Tabligh (propagation). Every single Muslim group unanimously concedes to every one of these points. If all the various groups are summoned to work on these points, they will al be prepared to participate without any objections. They will all be prepared to implement them. In other words, these points form the only platform on which members of ALL the Muslim groups can be seen united without raising objections against each other.
You must have witnessed that every movement in the world draws up certain policies which they refer to as their manifesto. None of the participating members object to these policies. Nobody asks why are they not ten instead of nine articles? Or why was no so and so issue included in it? So if the six points of Tabligh are singled out for criticism then the manifestos of every single movement should also attract attention. However, this is not the case. From this we deduce that it is not necessary to include everything. Nonetheless, the only question one may pose is that the policies you selected as your objective, are they being fulfilled? Alhamdulillah, these points are diligently worked upon and thereafter , step by step a person is promoted in deen depending on his capabilities.
In brief, these six points play a prominent role in uniting the various groups of deen and in developing the capability of implementing deen.