Admittedly most Catholics view the Holy Eucharist as similar or in par with that of Protestant service. In view of this mentality some Catholics were already attending Protestant service because they were being invited by their friends or family members. The Catholic Mass and Protestant service are greatly opposite of each other.
Protestant Service Focuses On
Emotional sermon of the Pastor
Loud and noisy music by the use of drums and electric guitar
Catholic Mass Focuses On
Solemnity and Sacredness
Transubstantiation of the Bread and Wine
Worship of the Holy Trinity
In Protestant service, aside from few instances where the congregation sings praises and worship 80% of the time is being consumed by the pastor in sharing his emotional sermon rather than actual worship of God. The attention is drawn more to the Pastor rather than to God and the songs were designed to be emotionally appealing rather than worshipful. Having this as the center of the Protestant service it is safe to say that they are not worshiping God, the preaching of the pastor by itself is not worship. If the sermon of the pastor constitutes 80% of the service it only means that they have more time in listening to their pastor rather than worshiping God.
This is very opposite to the worship of Catholics in the Holy Eucharist, at the beginning of the Mass we already offer our prayer to God through a song. All the gestures in the Mass also points to the sacredness of the Holy Eucharist and also to the presence of God in our midst. From the beginning until the end our focus is drawn to Christ rather than the priest. It is only through the Holy Eucharist where we can say that we are truly worshiping God.
In view of these facts Catholics are prohibited from attending Protestant service because it does not fulfill our Sunday obligation.
So, Venerable Brethren, it is clear why this Apostolic See has never allowed its subjects to take part in the assemblies of non-Catholics: for the union of Christians can only be promoted by promoting the return to the one true Church of Christ of those who are separated from it, for in the past they have unhappily left it. To the one true Church of Christ, we say, which is visible to all, and which is to remain, according to the will of its Author, exactly the same as He instituted it. Mortalium Animos No. 10, Pope Pius XI
Since the Protestant service was not ordained by God and did not originate from God it is not a valid form of worship. Catholics worship God the way God wants to be worship. On the other hand Protestants worship God the way they want their religious emotions be satisfied.